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

I'd extract this into a full-blown function; because it doesn't use any state from the enclosing scope (i.e. you pass in `header` explicitly), it doesn't need to be inlined. There's a probably-negligible performance gain to doing that because the function doesn't get redefined every time, and also makes it clear that you don't actually depend on any of the enclosing scope's state.

When you do that, though, I'd also add a docstring, and make it explicit what kind of object `header` is. We have many different types of objects that we call `headers`, and since this one has a weird flags-array-thing structure, it needs to be clear to the caller what kind of header must be passed in. Or maybe even calling it something like `isActiveSyncHeaderRepSeen()` would make that clearer.