Show delivery status for threaded messages
While discussing #651 (closed) we realised that a member of a private group might send a message to a dissolved group before finding out that the group had been dissolved. This raised the more general issue of showing delivery status for threaded messages.
We could reuse the clock (unsent) and single tick (sent) icons from private conversations, but the double tick (delivered) icon might not make sense for forums and private groups, where there are multiple recipients. For private groups, we might also need an icon or other indication for messages that weren't delivered to the group's creator before the group was dissolved.