Handle moved messages in unreads data

[gnprice]

With #787 we implemented most of #150, handling moved messages. But as that PR description said:

Fixes most of #150. (After this lands we should do a quick scan to identify anything else that needs updating on message moves — there are several new data structures in the store that we've merged in recent weeks.)

Then we didn't do that scan.

Places that still need updating on message moves include:

• Unreads (which has a TODO comment for it)
• RecentSenders (that's one of those recently-merged data structures) → split to its own issue: Handle moved messages in recent-senders data #1460
• … OK and I think that's it, having now scanned through what's in the store. (Another recently-merged data structure is RecentDmConversationsView, but that's only about DMs and those can't be moved.)

Related issues:

• Update topic input in channel compose box when topic moved #862

[gnprice]

The key user-facing scenario where this comes up, which I happened to see again earlier today:

• You're reading messages in Combined feed.
• A topic gets resolved, containing some unread messages.
• The messages are still unread, and shown with the blue unread marker at left. If you tap the recipient header to navigate to their conversation, the unread marker appears, as it should.
• But the conversation doesn't have a mark-as-read button. As a result there's no way for you to dismiss those unread markers (short of marking read the entire combined feed, or entire channel).

What's happening is that the Unreads data structure still has the messages under their old, unresolved topic; so when viewing the new resolved topic, it finds no unreads for that topic.

[PIG208]

I will work on this next as with #1301 this scenario might come up more often.