repo groups: make it possible to remove own explicit permissions, now when group owners always have admin permissions Until recently, group owners very given explicit admin permissions on repo group, and special care was taken to make sure they didn't remove themselves. Now we always give admin permissions to owners, and don't care about the explicit permissions. We no longer add them when creating groups or changing owner. There is no migration step to remove redundant permissions, but we should allow group admins to remove them. This change will thus remove the mechanism for preventing removal of own/owner permissions.