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
In general, we try to avoid using .innerHTML if we can (tmpl generation a notable exception due to how it works). In this case, it would be good to always just have the span in the DOM and either use the 'collapsed' class to hide and show it, or rely on empty content collapsing it.