Re-implementing the loading indicator for the Ember admin

closes #2855 , closes #2848
- New  mixin that utilizes NProgress for displaying a loading indictor for all routes who's model issue a "loading" event (aka: when requesting data from the server during a route change).
- Also removing (the now unnecessary) "loading" template.
Merge pull request #3024 from morficus/nprogress-loading-indicator

Re-implementing the loading indicator for the Ember admin