Handle moved messages (update_message events, 2/2)

[gnprice]

https://zulip.com/api/get-events#update_message

This issue is for the case where the messages were moved to a different stream or topic. (The case where a message's content changed is #118.)

In particular these should be handled in the MessageListView view-model, so that the message disappears from the view if it no longer belongs in that particular narrow, or appears if it newly does belong.