Add "single user mode"

While the instance is in single user mode only users in the admin group can use the instance.
This can give admins the time to configure or update the instance without regular users "getting in the way"

single user mode can be controlled by
- `occ maintenance:singleuser --on`
- `occ maintenance:singleuser --off`
- manually setting the `singleuser` option in config.php

cc @karlitschek