fix(client): The Sync flow should not notify the browser of login after the signup confirmation poll completes.

The browser polls in the background for the status of the sign up completion itself, the client does not need to notify it of completion.

In addition, the /confirm screen should not transition to the /signup_complete screen, this causes a jerky UI.

fixes #1881
      // The Sync broker notifies the browser of an unverified login
      // before the user has verified her email. This allows the user
      // to close the original tab or open the verification link in
      // the about:accounts tab and have Sync still successfully start.