Relocate RSVP configuration.

Ember.DeferredMixin will ultimately be deprecated and removed, but this
configuration (that allows test friendly promises) is not related to
Ember.DeferredMixin anyways.

This just moves that code out of the DeferredMixin.
Remove `.then` from Ember.Application.
Deprecate Ember.DeferredMixin and Ember.Deferred.
Deprecate `.then` on Ember.Application.
Merge pull request #5215 from rwjblue/deprecate-deferred

Deprecate Ember.DeferredMixin and Ember.Deferred.