Lock server so no new members can join

[ProgramPhoenix]

Is your feature request related to a problem? Please describe.
We currently cannot prevent randos from joining servers (apart from not naming the domain).

Describe the solution you'd like
I'd like to give the dm a button in the settings popup to enable people to join (toggle button). The default would be that no one can join and will be reset on a server restart. Also, when no user exists, the server allows automatically joining. The default option should be overwriteable by an environment variable

[Kr0nox]

Needs to Backend endpoints:

• Check the status of the lock
• Change the status of the lock

[ProgramPhoenix]

The endpoint are implemented here: https://github.com/NamelessGroup/InventarWerk/tree/lock-server