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

Corresponding PR in gelam is here: https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/315
I suppose we might still need a bind for the setTimeout, but also would want some more background on why we would need to adjust the margins in JS vs working out a spiffy HTML+CSS only solution. Maybe flexbox is the answer (I say without diving deep into the code).