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
The 999+ will need to be an l10n string lookup. See the 'compose-attachments' l10n entry in the locales/email.en-US.properties and how 'compose-attachments' is referenced in the JS for how to construct a value that can have pluralities.