Do not clear password until after leaving signin

Closes #3399
* Provide our own authenticate action handler which does not
  clear the password input.
* Use the Signin route's deactivate hook to clear the password
  property on the controller after the user has transitioned
  away from the signin page.
LoginController extends AuthenticationController and provides an `authenticate` action that does two things:  sends the credentials up the stack, and clears the password property on the controller.

Since we don't want that behavior we can just take LoginController out of loop and provide our own `authenticate` action.