Add an option to delete the local built-in group "admin"

Add an option to delete the local built-in group "admin": The NC setup creates a user "admin" and a group "admin". While the admin user can be deleted, the admin group cannot. Moreover, it seems that there are certain permissions checks which are hard-coded for the literal string admin, see bug #42480. On top there is already an option to delegate some administrative privileges to other groups via ./settings/admin/admindelegation.