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

Probably lose this comment here and just mention that this file is derived from test_folder_storage.js.  It's usually preferable to avoid duplicating any of my huge comment blocks to multiple places in the codebase since then we do run into the pitfalls of out-of-date documentation or inconsistent documentation as maybe only one of them gets updated/etc.