Don't auto-pick a v2 address when it's in Onion-Location header

As suggested by the discussion with @pastly on #tor just now:

In the upcoming Tor Browser, we aren't able to load v2 onion addresses anymore. But if you clicked "always redirect me" on the Onion-Location handler, then the browser automatically slams you into the ground whenever you visit a site that tells you a v2 address in its Onion-Location header.

Maybe that means we should decline to auto-follow v2 addresses?