Enable use for plain ruby classes

[jsugarman]

What

Allow creation of GDS design system form components for plain ruby classes. Issue.

Why

Inline with standard rails form builder helpers and to facilitate creation of simple forms with no models associated.

For example to create a simple search form it would be nice to not have to create any model and just do something like

# routes.rb
post 'search', action: :index, controller: :search

# app/controllers/search_controller.rb
class SearchController < ApplicationController
  def index
    @search_options = [[:opt1, 'Option 1'], [:opt2, 'Option 2']]
  end
end

# app/view/search/index.html.haml
= form_with url: search_path, local: true do |f|
  = f.govuk_collection_radio_buttons :search_option,
      @search_options,
      :first,
      :last,
      legend: { text: 'Choose a filter' },
      hint_text: 'Specify a filter for the search'
  = f.govuk_text_field(:query, label: { text: 'Find a thing' }, hint_text: 'enter thing to find' )
  = f.govuk_submit('Search')

Currently this is only prevented by the use of calls to errors, from what i can see.

[peteryates]

Overall this looks good. Before this was raised I'd already created #90 which does more or less the same thing. If you're happy for me to I'll cherry pick the tests from this PR into that one and tweak accordingly. Thanks @jsugarman

[peteryates]

I think the builder should error if the dev tries to render an error summary in a form with an object that doesn't support errors

[jsugarman]

Overall this looks good. Before this was raised I'd already created #90 which does more or less the same thing. If you're happy for me to I'll cherry pick the tests from this PR into that one and tweak accordingly. Thanks @jsugarman

Yep, what ever you think is best.

[jsugarman]

I think the builder should error if the dev tries to render an error summary in a form with an object that doesn't support errors

👍 Makes sense, better to catch these things early.