Inform about Maintenance Mode in Admin Login Panel

When trying to log in to the administration while the maintenance mode is active, the user is left with a blank screen:

This happens because the maintenance mode disables Ajax which prevents loading the login control.

There should be an info that the maintenance mode is active and therefore logging in isn't possible at the moment.

Furthermore an option to force a login in maintenance mode should be considered. I imagine a user which has no CLI access to his QUIQQER system. If he actives maintenance mode and then gets kicked out of the administration (for whatever reason). He is locked out of his system.