refs https://github.com/TryGhost/Admin/pull/1901
Ember has deprecated mixins in preparation for 4.0 and `ember-simple-auth` has now done the same in 3.1.0.
- removed all imports of Ember Simple Auth mixins
- moved authenticated and invalidated handling from application route to session service
- moved server-notification loading from application route to session service
- updated `AuthenticatedRoute` to use the session service directly rather than authenticated route mixin
- added `UnauthenticatedRoute` that incorporates the behaviour from our overridden `UnauthenticatedRouteMixin` and switches to using the session service directly
