Fire NProgress on User, Post, and Settings save

Closes #3037
- Created `NProgressSaveMixin`, which extends the `save` method of a model to fire NProgress.
- Extended `UserModel`, `PostModel`, and `SettingModel` with the new mixin.
Nope, just personal style. I think that `Ember.RSVP.reject` is more consistent with other places in the codebase; I'll update