Fetch install/update metadata files from a different location/

The Tor Browser people decided to randomly change the URL that contains the update XML files, so metadata will live under update_3 at some point in the future. The sandbox code pulls all channels pull updates from update_2 at the moment (along with the relevant downloads.json files).

There's some talk of redirects so as long as update_2 keeps working till something like 7.5, this can probably be pushed off till 7.0 ships.

See: https://trac.torproject.org/projects/tor/ticket/19316#comment:20

(It would be nice if, they told me they were doing this, though I guess stumbling on it randomly in trac works too.)