Perform TBB version check async on new tab

Erinn was concerned that the async version check was not happening at the right points for a substantial segment of the userbase.

She suggested doing it on on new tab, too. We can do this, but then we'd have to do something crazy like pop up an out-of-date notice then, too.

The phishing risk related to this could be a nightmare, though. We do not want to train our userbase to follow urls from random popups instructing them to download binaries...