Store message count, unread count and timestamp of latest message in group metadata
requested to merge 584-store-latest-timestamp-and-unread-count-in-group-metadata-for-private-messaging into master
This is to eventually address #373 (closed) and slowness of other lists. The group metadata is not yet used, but if this MR isn't merged fast, another commit that actually uses it and thus takes care of the slowness will be added.
Closes #584 (closed), #585 (closed), #586 (closed)