Bug 800402 - [email] UI: Show the number of unread messages in the message-list header after the folder name. back-end fix-up. r=asuth

propagating the GELAM fix:
https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/330
Merge pull request #23132 from asutherland/email-unread-msgs-fix

Bug 800402 - [email] UI: Show the number of unread messages in the message-list header after the folder name. back-end fix-up. r=asuth