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

I think this version is semantically really only for the FolderStorage internals so we want the naming and comment to better reflect that.  Also, I think it makes more sense to stash the constant closer to the code we'd be changing that would need to affect this version.  So I propose we store and export the constant in mailslice.js (which the other references are already requiring) and mayb ecall it something like FOLDER_DB_VERSION.