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

We can avoid the horrible code smell of having this huge chunk of copied and pasted code by leveraging our awesome module system.  Specifically, we can refactor the common stuff into a 'shared_folder_storage.js' file that both things require and then suck the stuff out of there.  I did this recently for outbox.  I also recently undid it because then I moved it into th_main, but you can see the commit where I added it in https://github.com/mozilla-b2g/gaia-email-libs-and-more/commit/d981ddcc554ad1bbc30ab5b3a98efecc64fbde69#diff-6dd737234ca9222ab2b006f2cdb47bf8R8 search on 'shared_outbox' in general to find the file and its consumers.