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.
If I'm not mistaken I think we should be returning a resolved or rejected promise here.  `DS.Model#save` returns a promise, so if we don't the calling semantics for saving a record will be changed, i.e. `record.save().then()`.

There's also some existing code that relies on save returning a promise for a model on success or a promise for an error object on failure so I think we've got to keep doing that...