Use a mixin for shared behaviour in services

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`.

This mixin replaces the inheritance approach used in services with an
ApplicationService class. These classes now all utilise this mixin
instead.

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 [1] 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 [2] 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 [3]. 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/rubocop-govuk#125
[2]: rubocop/rubocop#8506
[3]: https://thoughtbot.com/blog/back-to-basics-solid#liskov-substitution-principle

app/services/assign_edition_status_service.rb

@@ -1,4 +1,6 @@
-class AssignEditionStatusService < ApplicationService
+class AssignEditionStatusService
+  include Callable
+
   def initialize(edition,
                  state:,
                  user: nil,

app/services/create_document_service.rb

@@ -1,4 +1,6 @@
-class CreateDocumentService < ApplicationService
+class CreateDocumentService
+  include Callable
+
   def initialize(document_type_id:,
                  content_id: SecureRandom.uuid,
                  locale: "en",

app/services/create_file_attachment_blob_service.rb

@@ -1,4 +1,6 @@
-class CreateFileAttachmentBlobService < ApplicationService
+class CreateFileAttachmentBlobService
+  include Callable
+
   def initialize(file:, filename:, user: nil)
     @file = file
     @filename = filename

app/services/create_image_blob_service.rb

@@ -1,6 +1,8 @@
 require "mini_magick"
 
-class CreateImageBlobService < ApplicationService
+class CreateImageBlobService
+  include Callable
+
   def initialize(temp_image:, filename:, user: nil)
     @temp_image = temp_image
     @filename = filename

app/services/create_next_edition_service.rb

@@ -1,4 +1,6 @@
-class CreateNextEditionService < ApplicationService
+class CreateNextEditionService
+  include Callable
+
   def initialize(current_edition:, user:, discarded_edition: nil)
     @current_edition = current_edition
     @user = user

app/services/delete_draft_assets_service.rb

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

app/services/discard_draft_edition_service.rb

@@ -1,4 +1,6 @@
-class DiscardDraftEditionService < ApplicationService
+class DiscardDraftEditionService
+  include Callable
+
   def initialize(edition, user, **)
     @edition = edition
     @user = user

app/services/discard_path_reservations_service.rb

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

app/services/edit_draft_edition_service.rb

@@ -1,4 +1,6 @@
-class EditDraftEditionService < ApplicationService
+class EditDraftEditionService
+  include Callable
+
   def initialize(edition, user, **attributes)
     @edition = edition
     @user = user

app/services/failsafe_draft_preview_service.rb

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

app/services/generate_base_path_service.rb

@@ -1,4 +1,6 @@
-class GenerateBasePathService < ApplicationService
+class GenerateBasePathService
+  include Callable
+
   def initialize(edition, title:, max_repeated_titles: 1000)
     @edition = edition
     @title = title.to_s

app/services/generate_unique_filename_service.rb

@@ -1,4 +1,5 @@
-class GenerateUniqueFilenameService < ApplicationService
+class GenerateUniqueFilenameService
+  include Callable
   MAX_LENGTH = 65
 
   def initialize(filename:, existing_filenames:)

app/services/preview_asset_service.rb

@@ -1,4 +1,6 @@
-class PreviewAssetService < ApplicationService
+class PreviewAssetService
+  include Callable
+
   def initialize(edition, asset, **)
     @edition = edition
     @asset = asset

app/services/preview_draft_edition_service.rb

@@ -1,4 +1,6 @@
-class PreviewDraftEditionService < ApplicationService
+class PreviewDraftEditionService
+  include Callable
+
   def initialize(edition, republish: false)
     @edition = edition
     @republish = republish

app/services/publish_assets_service.rb

@@ -1,4 +1,6 @@
-class PublishAssetsService < ApplicationService
+class PublishAssetsService
+  include Callable
+
   def initialize(edition, superseded_edition: nil)
     @edition = edition
     @superseded_edition = superseded_edition

app/services/publish_draft_edition_service.rb

@@ -1,4 +1,6 @@
-class PublishDraftEditionService < ApplicationService
+class PublishDraftEditionService
+  include Callable
+
   def initialize(edition, user, with_review:)
     @edition = edition
     @user = user

app/services/remove_document_service.rb

@@ -1,4 +1,6 @@
-class RemoveDocumentService < ApplicationService
+class RemoveDocumentService
+  include Callable
+
   def initialize(edition, removal, user: nil)
     @edition = edition
     @removal = removal

app/services/rescue_scheduled_publishing_service.rb

@@ -1,4 +1,6 @@
-class RescueScheduledPublishingService < ApplicationService
+class RescueScheduledPublishingService
+  include Callable
+
   def initialize(edition_id:)
     @edition_id = edition_id
   end

app/services/resync_document_service.rb

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

app/services/schedule_publish_service.rb

@@ -1,4 +1,6 @@
-class SchedulePublishService < ApplicationService
+class SchedulePublishService
+  include Callable
+
   def initialize(edition, user, scheduling, **)
     @edition = edition
     @user = user

app/services/withdraw_document_service.rb

@@ -1,4 +1,6 @@
-class WithdrawDocumentService < ApplicationService
+class WithdrawDocumentService
+  include Callable
+
   def initialize(edition, user, public_explanation:)
     @edition = edition
     @public_explanation = public_explanation

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(*args, **kwargs, &block)
+      new(*args, **kwargs, &block).call
+    end
+
+    private_class_method :new
+  end
+end