Bug 800402 - [email] UI: Show the number of unread messages in the message-list header after the folder name

I think you also need an incrementing variant of this in the remove case.  And you'll want a little more test coverage for this too.  Note that when we do the undo operation in test_mutation it effectively flips adds to removes and vice versa, so if it seems like there isn't existing coverage, that's where it is.  (Or at least is one of the places you can find it.)