Handle live updates to whether messages should appear in the @-mentions narrow

[PIG208]

We should handle UpdateMessageEvent so that when the message becomes visible to the @-mentions narrow, it is added to the MessageListView, and similarly when the reverse happens.

These can occur when a message is edited to add or remove an @-mention. The event contains the new flags value, which gives us the information we need.

Currently the web app does not handle this, so we are not pressed to do so.

This is a follow-up to:

• msglist: Offer @-mentions narrow #250