Fixed: the parcel flag "Allow Scripts from Group" should only check if the parcel has a Group set; it doesn't have to be *deeded* to the group

Also some cleanup of the use of Group ID's (with no change to functionality).
Fixed: if a user belonged to the parcel's group then he was allowed to rez object in the parcel even if the flag "Create Objects by Group" was disabled
Fixed: once a user has rezzed an object, they could then duplicate it as much as they wanted even if the parcel's permissions had since been changed to disallow rezzing
Fixed: when any avatar changed his Active Group, it was set as the active group for *all* the avatars in the scene (not permanently)
Stopped setting the Service URL "GatekeeperURI" on users' accounts. It isn't actually used.
Better error-handling when storing assets: recognize that 'null' is an error value