Provide an actual maintenance mode

As a site admin I'd like to put my Backdrop sites in a 'real' maintenance mode when there is a critical security release and I don't have time to update all sites immediately.

The 'real' or 'actual' maintenance mode (or "vacation mode"?) would display a static page / a static version of the site and make it so that Backdrop isn't running anymore. That's a difference to the current maintenance mode where Backdrop continues to run, for instance to provide the login form. (I guess there are also other active functionalities in maintenance mode.)

Speaking of the login form: we need a way to login to a site which is in vacation mode. This could be implemented via settings.php.