* Moved message deletion into the CtdlRoomAccess() API. The
old function DoIHavePermissionToDeleteMessagesFromThisRoom() is now a
simple wrapper around CtdlRoomAccess().
* This gives us the ability to query arbitrary room/user combinations, not
just "this user, this room." It also paves the way for the ability to
grant "delete privileges" to any user for any room in the future.
* IMAP ACL now uses this feature to set or clear the "t" flag. Observed
that Thunderbird now activates or deactivates the Delete button for each
folder based on the "t" flag.