Rewrite the login form with FormUI. Add a new admin formcontrol for password fields. Use it. This will break plugins relying on theme_loginform_* hooks. Also, it requires rewriting of the login handler. Handles #461
Turn the old login handler into a new one for FormUI. Use it.
Move the new login FormUI into UserHandler and get rid of the ancient stuff there.
Let plugins alter the new login FormUI. Fixes #461