Add newly-starred messages live when viewing starred-messages narrow

[gnprice]

This is like #818, but for StarredMessagesNarrow and a message getting starred, via UpdateMessageFlagsEvent.

On the other hand when a message gets unstarred, we don't want to remove it from view if the user is looking at the starred-messages narrow. Leaving it in view is handy as an "undo" mechanism.

[gnprice]

In zulip-mobile we do this but there's a bug where if the newly-starred message is one that we don't already have locally on the client, we breach an invariant by adding a dangling message ID to the list of starred messages:

• In the starred messages view, new starred message events crash the app zulip-mobile#5895

I think we'll naturally avoid any bug like that when implementing this functionality in zulip-flutter, because MessageListView carries a list of Message objects rather than just a list of message IDs. (This ultimately comes down to the fact that zulip-flutter's data structures for the Zulip data model are generally mutable, whereas they're immutable in zulip-mobile.) But in any case, that bug makes a reminder to systematically handle all possible cases when we implement this.