"Check for Tor Browser Update" label should by dynamic

As discussed on IRC, it is a little irritating (and potentially confusing) that Tor Browser displays "WARNING: this browser is out of date" but then directs users to choose "Check for Tor Browser Update" (clearly, the browser already knows that there is an update available).

We could change the text of that Torbutton menu item to "Update Tor Browser" in this situation or we could go even further and add an "Update Now" button to the about:tor page.