Contentable Delegated Type

[A-Mozeak]

Resolves #107

Description

This PR adds a Contentable delegated type that allows Games, Links, and future items to be attached to a ContentModule.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• test/controllers/content_modules_controller_test.rb
• test/controllers/games_controller_test.rb
• test/models/content_module_test.rb
• test/models/content_test.rb

[A-Mozeak]

@sean-dickinson mentioned that it'd be good to move Content#destroy_owned_record into Contentable, making it possible to give the assorted content types a default behavior when destroying a ContentModule. I agree, and we'll lean false (i.e. nullable) for the default behavior.