Update recent-DMs data on fetching messages

[gnprice]

When a new message comes in, we update the RecentDmConversationsView data structure. But when we go and fetch a bunch of messages for a message list, we don't.

Mostly this works out, because the initial snapshot has an unreads digest that already covers the most recent DM conversations, up to a large number of recent messages. But if you go look at some history that includes a long-inactive DM conversation —say, you follow a link to such a conversation — we'll end up with a DM conversation we have some messages from but that doesn't appear in the list of recent DM conversations, in particular the corresponding screen in the UI. That's odd and would be good to fix.

This will most naturally come after a central message store, #648 / #455.

Similar to #649. This issue and that one can be done in any order.

Milestoned post-launch; we do currently do this in zulip-mobile, but I suspect few users will notice if we don't.