[Feature Request]: Maintenance mode and online users counter

[swift2512]

Is your feature request related to a problem? Please describe.

I can't find how to put SnipeIT in maintenance mode (couldn't find anything related in documentation) to prevent users from editing stuff while backups are made, system is being upgraded. Also, there's no counter with information how many users are logged in.

Describe the solution you'd like

Superuser should be able to set maintance mode via GUI (not everyone has access to server to edit .env file) and put a system in read-only mode to make backups, imports, system version upgrades, etc. All other regular and admin users would be able to login, but couldn't change anything - they would see system maintenance in progress banner at the top of the page.

Describe alternatives you've considered

If read-only mode not possible or too difficult to implement, system should allow everyone to login, but great non superusers with maintenance in progress page and a link to logout. Superusers could use system without any restrictions.

Additional context

No response