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.
Since `ValidationEngine` is also overriding `save` and I believe we only want NProgress to do its thing on network-related activities, if you switch the order of the mixins here, `NProgressSaveMixin#save` will only get called if the validations pass and the model is ready to be sent to the API.