unreadModel: respond to message topic/stream edits

[WesleyAC]

We currently don't update unreads when a message topic is edited. We should do so.

See CZO discussion.

[gnprice]

Similarly when a message is moved between streams.

Both of these changes come to us in the form of update_message events:
https://zulip.com/api/get-events#update_message
We turn those events into Redux actions with type EVENT_UPDATE_MESSAGE.

So the place to implement this will be in src/unread/unreadModel.js, in streamsReducer, which should get a case EVENT_UPDATE_MESSAGE: section.

[gnprice]

This causes pretty frustrating behavior when it happens: there's a thread in the unreads list, you tap on it to open, and then we fetch and find no messages, producing a nonsensical screen like this (reported in chat; discussion below that from about here):

Moreover, thanks to topic resolving, it's now a pretty common occurrence for a topic to get renamed, where once it was an edge case.