[SPIKE] Use Simple Form to build GOV.UK Frontend-ready forms

[tijmenb]

This builds on top of #331.

Simple Form allows you to build forms using a simple DSL:

<%= simple_form_for @user do |f| %>
  <%= f.input :username %>
  <%= f.input :password %>
  <%= f.button :submit %>
<% end %>

We are already using Simple Form in contacts admin and content tagger. We also have a standard configuration for Simple Form in govuk_admin_template.

This PR attempts to create a configuration file that uses GOV.UK Frontend. If it works, it will allow developers to forms with the correct markup without knowing anything about GOV.UK Frontend. Existing forms in content tagger might even be converted to use the new styling simply by switching out the config.

Things that "just work"

• Text input
• Textarea
• Submit button
• Select box
• Single check box (boolean)
• List of checkboxes
• List of radio buttons
• Hints

Things that don't work yet

• Errors (still to do)
• The Date input component
• Fieldset & legend for the radio & select boxes
• General spacing
• Avoid having to @extend the checkbox & radio classes

Screenshot

https://govuk-publishing-compon-pr-336.herokuapp.com/admin/form

https://trello.com/c/1S9peN8b/62-spike-a-layout-system-for-admin-apps