Moderators can change groups such that they lose access

[Steradiant (Maarten)]

This bug is quite similar to

Moderators can create/change categories such that they lose access

There is some setting which allows moderators to manage categories. If this is activated, moderators can change the security settings of a category such that they no longer have access, which is most likely accidental. Administrators can, of course, recover this situation. I suggest to remedy this behavior by forbidding moderators to change a categorry in such a way that they loose access.

There is some setting which allows moderators to manage groups.
If this is activated, moderators can change the security settings of a groups such that they no longer have access, which is possibly accidental.
Administrators can, of course, recover this situation.

Furthermore, there is an additional odd situation regarding the access of the member list, where Moderators can toggle whether they themselves see the member list.
But maybe that’s not too much of a problem.

[zogstrip]

A warning (confirm dialog) will be shown once this PR gets merged

FIX: Warn moderators before group changes that would lock them out