Transition user to original destination on login

Closes #2943

- `AuthenticatedRoute` now stores the user's original destination on the
  `ApplicationController`.
- The `SignIn` route's `login` action checks if the
  `originalUserDestination` property is set on the
  `ApplicationController`, and if so, retries that transition after a
  successful login.


I definitely jshint'd this before PRing. Go me.