Transition user to original destination after signin

Closes #2943
- `AuthenticatedRoute` now stores the user's original destination on the
  `ApplicationController`.
- The `SignIn` route's `login` action checks if the `loginTransition` property is set on the
  `ApplicationController`, and if so, retries that transition after a
  successful login.
Merge pull request #2952 from novaugust/ember-signin-transition

Transition user to original destination on login