Prevent the removal of the last admin per meeting

[Elblinator]

Current behaviour:
It is possible to have a meeting without any user as it's admin (specifically the admin group here)

This can lead to problems for the users and after the feature to lock a meeting from the inside, it can even lead to inaccessible meetings

Known ways to remove the last admin:

• As superadmin/orgaadmin: remove/delete the last admin (possible from orga- and meeting-level)
• As accountadmin/committeadmin: delete the last admin (possible from orga-level)
• As "meeting" admin: remove the admin group from everyone (including yourself)

Wanted behaviour:
If someone tries to remove the last admin from a meeting this action should be blocked and thrown an error

Any meeting which already has no user in the admin group should stay like that. -> meaning no migration is needed

[luisa-beerboom]

We should also probably pay attention to being locked out of meetings

[luisa-beerboom]

It should still be possible to remove the last admin from template meetings, it should not be possible to remove template status from a template meeting without admins