Use a mixin for common .call class interface

This creates a new mixin, Callable, which can be used to provide a class
with the boilerplate so that it can have a public class method of
`.call` which calls `#initialize` with the arguments and then `#call`.

The creation of this class is a port of the same approach applied in
Content Publisher [1] (and most of this commit message is a port too in
case you're feeling a touch of déjà vu).

This mixin replaces the inheritance approach used in builders,
presenters and services.

The change was prompted by a linting violation with one of rubocop's newer
rules: `Lint/MissingSuper`, which is an offence when a class inherits but
does not call `super` in it's initialize method. This violation seemed
to only affect some of GOV.UK's newer code, such as this service
pattern, which led to some debate [2] about what the most
appropriate means to share this behaviour is, whether our
inheritance approach was actually idiomatic and how using super in every
`initialize` would be unnecessary boilerplate.

The Rubocop thread [3] regarding this rule raised an interesting point
"I'm curious of examples where there are good reasons to explicitly not
call super.", which led to me considering why we didn't think `super`
would be appropriate in these classes. My conclusion to this was that we
weren't actually intending to create a subtype with inheritance, we
actually just wanted standardisation with the interface, and that the
inherited classes have very little behavioural consistency, breaking
Liskov substitution principle [4]. When looking at the Ruby standard
library a module seems the most appropriate way to implement it,
considering similarities with Singleton and Enumerable.

[1]: alphagov/content-publisher@5933f87
[2]: alphagov/rubocop-govuk#125
[3]: rubocop/rubocop#8506
[4]: https://thoughtbot.com/blog/back-to-basics-solid#liskov-substitution-principle

app/builders/bulk_subscriber_list_email_builder.rb

@@ -1,4 +1,6 @@
-class BulkSubscriberListEmailBuilder < ApplicationBuilder
+class BulkSubscriberListEmailBuilder
+  include Callable
+
   BATCH_SIZE = 5000
 
   def initialize(subject:, body:, subscriber_lists:)

app/builders/digest_email_builder.rb

@@ -1,4 +1,6 @@
-class DigestEmailBuilder < ApplicationBuilder
+class DigestEmailBuilder
+  include Callable
+
   def initialize(content:, subscription:)
     @content = content
     @subscription = subscription

app/builders/immediate_email_builder.rb

@@ -1,4 +1,6 @@
-class ImmediateEmailBuilder < ApplicationBuilder
+class ImmediateEmailBuilder
+  include Callable
+
   def initialize(content, subscriptions)
     @content = content
     @subscriptions = subscriptions

app/builders/subscriber_auth_email_builder.rb

@@ -1,4 +1,6 @@
-class SubscriberAuthEmailBuilder < ApplicationBuilder
+class SubscriberAuthEmailBuilder
+  include Callable
+
   def initialize(subscriber:, destination:, token:)
     @subscriber = subscriber
     @destination = destination

app/builders/subscription_auth_email_builder.rb

@@ -1,4 +1,6 @@
-class SubscriptionAuthEmailBuilder < ApplicationBuilder
+class SubscriptionAuthEmailBuilder
+  include Callable
+
   def initialize(address:, token:, subscriber_list:, frequency:)
     @address = address
     @token = token

app/builders/subscription_confirmation_email_builder.rb

@@ -1,4 +1,6 @@
-class SubscriptionConfirmationEmailBuilder < ApplicationBuilder
+class SubscriptionConfirmationEmailBuilder
+  include Callable
+
   def initialize(subscription:)
     @subscription = subscription
     @subscriber = subscription.subscriber

app/presenters/bulk_email_body_presenter.rb

@@ -1,4 +1,6 @@
-class BulkEmailBodyPresenter < ApplicationPresenter
+class BulkEmailBodyPresenter
+  include Callable
+
   def initialize(body, subscriber_list)
     @body = body
     @subscriber_list = subscriber_list

app/presenters/content_change_presenter.rb

@@ -1,6 +1,8 @@
 require "redcarpet/render_strip"
 
-class ContentChangePresenter < ApplicationPresenter
+class ContentChangePresenter
+  include Callable
+
   EMAIL_DATE_FORMAT = "%l:%M%P, %-d %B %Y".freeze
 
   def initialize(content_change, subscription)

app/presenters/footer_presenter.rb

@@ -1,4 +1,6 @@
-class FooterPresenter < ApplicationPresenter
+class FooterPresenter
+  include Callable
+
   def initialize(subscriber, subscription)
     @subscription = subscription
     @subscriber = subscriber

app/presenters/message_presenter.rb

@@ -1,4 +1,6 @@
-class MessagePresenter < ApplicationPresenter
+class MessagePresenter
+  include Callable
+
   def initialize(message, _subscription = nil)
     @message = message
   end

app/presenters/source_url_presenter.rb

@@ -1,4 +1,6 @@
-class SourceUrlPresenter < ApplicationPresenter
+class SourceUrlPresenter
+  include Callable
+
   def initialize(url, utm_source:, utm_content:)
     @url = url
     @utm_source = utm_source

app/services/auth_token_generator_service.rb

@@ -1,4 +1,6 @@
-class AuthTokenGeneratorService < ApplicationService
+class AuthTokenGeneratorService
+  include Callable
+
   CIPHER = "aes-256-gcm".freeze
   OPTIONS = { cipher: CIPHER, serializer: JSON }.freeze
 

app/services/content_change_handler_service.rb

@@ -1,4 +1,6 @@
-class ContentChangeHandlerService < ApplicationService
+class ContentChangeHandlerService
+  include Callable
+
   def initialize(params:, govuk_request_id:, user: nil)
     @params = params
     @govuk_request_id = govuk_request_id

app/services/create_subscriber_list_service.rb

@@ -1,4 +1,6 @@
-class CreateSubscriberListService < ApplicationService
+class CreateSubscriberListService
+  include Callable
+
   def initialize(title:, url:, matching_criteria:, user:)
     @title = title
     @url = url

app/services/create_subscription_service.rb

@@ -1,4 +1,6 @@
-class CreateSubscriptionService < ApplicationService
+class CreateSubscriptionService
+  include Callable
+
   attr_reader :subscriber_list, :subscriber, :frequency, :current_user
 
   def initialize(subscriber_list, subscriber, frequency, current_user)

app/services/digest_initiator_service.rb

@@ -1,4 +1,6 @@
-class DigestInitiatorService < ApplicationService
+class DigestInitiatorService
+  include Callable
+
   def initialize(date:, range:)
     @range = range
     @date = date

app/services/immediate_email_generation_service.rb

@@ -1,4 +1,6 @@
-class ImmediateEmailGenerationService < ApplicationService
+class ImmediateEmailGenerationService
+  include Callable
+
   BATCH_SIZE = 5000
 
   def initialize(content, **)

app/services/matched_content_change_generation_service.rb

@@ -1,4 +1,6 @@
-class MatchedContentChangeGenerationService < ApplicationService
+class MatchedContentChangeGenerationService
+  include Callable
+
   def initialize(content_change, **)
     @content_change = content_change
   end

app/services/matched_message_generation_service.rb

@@ -1,4 +1,6 @@
-class MatchedMessageGenerationService < ApplicationService
+class MatchedMessageGenerationService
+  include Callable
+
   def initialize(message, **)
     @message = message
   end

app/services/message_handler_service.rb

@@ -1,4 +1,6 @@
-class MessageHandlerService < ApplicationService
+class MessageHandlerService
+  include Callable
+
   def initialize(params:, govuk_request_id:, user: nil)
     @params = params
     @govuk_request_id = govuk_request_id

app/services/send_email_service.rb

@@ -1,4 +1,6 @@
-class SendEmailService < ApplicationService
+class SendEmailService
+  include Callable
+
   class NotifyCommunicationFailure < RuntimeError; end
 
   def initialize(email:, metrics: {})

app/services/unsubscribe_all_service.rb

@@ -1,4 +1,6 @@
-class UnsubscribeAllService < ApplicationService
+class UnsubscribeAllService
+  include Callable
+
   attr_reader :subscriber, :reason
 
   def initialize(subscriber, reason, **)

lib/callable.rb

@@ -0,0 +1,14 @@
+# This mixin is to provide the boilerplate for classes that implement a single
+# public class method of `.call` - typically used by objects that are entirely
+# to perform a business transaction.
+module Callable
+  extend ActiveSupport::Concern
+
+  included do
+    def self.call(...)
+      new(...).call
+    end
+
+    private_class_method :new
+  end
+end