Removing use of subclassed application constant and replacing with Rails.application

The use of ``Rails.application`` instead of calling the application constant is a more agnostic syntax. This means tests will be more portable, and won't rely on the existence of a particular subclass. I've changed a bunch of places to use the ``Rails.application`` syntax.

By not calling the ``AppTemplate::Application`` constant, in the future it will be easier to have multiple applications or have applications be given by a particular instance of an ``Application`` instead of a subclass.