Create a generic 'maintenance mode' page/vhost

[pm98zz-c]

While we use Drupal's built in maintenance mode, this isn't suitable for other types of projects. It would be instead easier to use a 'flat' html page (path overridable in config.ini with a generic default (?)), that gets switched off/on, presumably at the vhost level.

This would make the deployment process suitable for any kind of stack, and prevent having to cope up with any Drupal changes going forward.