Add ModelAdminDecorator class and subtypes

ModelAdminDecorator is responsible for bolting view logic onto models.
ModelAdminController will work with ModelAdminDecorator subtypes as
opposed to Eloquent subtypes.
Modify ModelAdminController to work with ModelAdminDecorator

This replaces controller logic that worked directly on the model.
Modify ModelAdminController subtypes

They now coincide with modifications made to their base class.