Require belongs_to associations by default

[anthonycrumley]

As a developer, I want config.active_record.belongs_to_required_by_default = true so that the data integrity of associations will be better.

This setting is found in config/application.rb. To be able to set this to true, the following will need to happen.

• Fix all the tests that are failing by providing values for all the belongs_to associations.
• Determine if any of the belongs_to associations are not required and mark them so.