Enhancements to Groups-as-moderators

This puts the "finishing touches" on groups-as-moderators feature:

* Warns users when changing a group's visibility to hidden
* Warns users when changing a group's type to "post based"
* Removes moderator groups from boards when the visibility is changed to hidden
* Removes moderator groups from boards when the type is set to "post based"
* Removes moderator groups from boards when the group itself is deleted (somehow I forgot that...)