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
Generally, no need to explicitly specify the font family for individual selectors. In this case in particular, Fira Sans will be used by default if it is installed, so definitely not needed.