Update message list on subscribe/unsubscribe events

[gnprice]

If the user unsubscribes from a channel/stream, then the combined feed will start treating the channel as muted. We should therefore update the message list when a SubscriptionRemoveEvent comes in (unless the channel was already muted), similar to what's called for on UserTopicEvent in #421.

Similarly in the reverse direction, on a SubscriptionAddEvent.

I'm giving this a late milestone because this isn't a situation users are likely to frequently see. It's not super frequent in the first place to subscribe or unsubscribe from a channel; and, crucially, there isn't any UI to do so on the combined-feed message list itself. So unlike #421 this isn't part of making the screen around any given piece of UI responsive to the user's actions on that UI.

Originally pointed out by @chrisbobbe at #421 (comment).