Register publishes a *classes.LoginGates registry. POST /login consults
it through controllers.CheckLoginGates after the password, activation and
throttle checks and the bcrypt rehash, and before the restore of a
deactivated account, last_seen and the mint: a gate result replaces the
token response, a gate error answers the opaque 500. The restore and its
reactivate mail now run after the gates, so a password alone cannot
reactivate an account a second factor protects. Without a gate /login
answers as before; refresh never consults the gates.
classes.LoginEvent (golem15.user.login) fires after every password login
that minted a token; controllers.FireLoginEvent lets other plugins fire it.
