Group creator should cease receiving moderation notices after leaving group

[mattdricker]

The creator of a private group is also the moderator, and receives notifications when an annotations are flagged as inappropriate.

If the group creator/owner later leaves the group there is no mechanism for transferring ownership to another user. As such, the original owner still receives moderation notifications when annotations are flagged even after they are no longer a member.

These notification are likely unwanted, and the former owner is probably no longer the appropriate user to make moderation decisions.