Split up the Page class into a non-auth and an auth one

This means non auth pages no longer require to simply ignore the
user_id. Next to that the user_id in the AuthPage is replaced by
the HeaderVerifier. Pages should use getUserId() to retreive the
user_id from now.

Signed-off-by: Toon Schoenmakers <nighteyes1993@gmail.com>
Added a gcm flag for consumers

This enables us to restrict the android/register and android/unregister
to our Android client. Third party apps should not be using it after all.

Signed-off-by: Toon Schoenmakers <nighteyes1993@gmail.com>