Prevent the last owner of a group from being able to delete themselves by 'adding' themselves as a master

Replaces !1708 (closed).

Fixes #1111 (closed).

mentioned in merge request !1708 (closed)

Reassigned to @rspeicher

Status changed to merged

mentioned in commit d431f3ec

Picked into 8-2-stable.

mentioned in commit 8a1cafeb