Merge branch...

Merge branch '584-store-latest-timestamp-and-unread-count-in-group-metadata-for-private-messaging' into 'master'

Store message count, unread count and timestamp of latest message in group metadata

This is to eventually address #373 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, #585, #586

See merge request !336