Add Form Models

by mPeter for Ruby on Rails

Replace the current combination of fields_for / accepts_nested_attributes_for helpers that create nested objects with HTML forms. Introduce a more object-oriented approach which allows us to create form objects that represents our form, instead of using the ORM internals, and place it inside the ActiveModel layer. These objects must conform to the ActiveModel API, so they can be used with existing form helpers such a form_for and deal with nested objects in a more seamless way.