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

Maybe add a comment for now that references the explicit list that lives in mailapi.js on the MailFolder docs.  Or if you can figure out how to define a JSDoc enum http://usejsdoc.org/tags-enum.html or something like that, we can clean that up right now.  My main concern is that we know there's a list somewhere and there's only one list.