Update unreads data on fetching messages

[gnprice]

When a new message comes in, we update the Unreads data structure, store.unreads. 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 unreads, up to a large number of them. But if you go look at some very old history where you have unreads, we'll end up with messages we know are unread but store.unreads doesn't reflect it. This is odd and would be good to fix.

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

Milestoned post-launch because we don't currently do this in zulip-mobile.