Generator should drop model classes in app/models, not lib/

[gburgett]

Currently the menu generator drops files in lib/wcc/contentful/model which reopen the WCC::Contentful::Model::Menu class to add functionality. These ought to be put in app/models as classes which inherit from the WCC::Contentful::Model::x classes. Example:

# app/models/menu_button.rb
class MenuButton < WCC::Contentful::Model::MenuButton
  # Override the "page" link validation, because the content type ID in JTJ
  # for pages has a capital P.
  validate_field :link, :Link, link_to: 'Page'

  ...
end

This is as opposed to the current usage which reopens the class:

# lib/wcc/contentful/model/menu_button.rb
class WCC::Contentful::Model::MenuButton < WCC::Contentful::Model

Potential complications:

• Validation logic (i.e. validate_field) stores validations in a class instance variable @field_validations - need to combine these from multiple subclasses.

• We need to require these models inside the wcc-contentful gem's init! method before we run validate_models! so that we can get the extra per-project validations (see example)

• button1 = MenuButton.find(id) vs button2 = top_nav_menu.buttons[0]
  Even though they represent the same thing in Contentful, right now button2
  would be an instance of WCC::Contentful::Model::MenuButton not of MenuButton.
  What happens if we try to compare these objects?