Indicate alt-svc onion explicitly in the urlbar

One of our initial debates over "should you use Onion-Location to explicitly send the user to the onion address" vs "should you use alt-svc to implicitly send the user to the onion address while leaving the url bar alone" was: if you leave it all implicit, the user never learns that it's happening. The downsides there are that the user never learns what an onion address is (and that it's not so scary), and also it's hard as a user to tell when or whether the browser actually switched to using the onion.

At the same time, Cloudflare and Facebook both made the choice to use alt-svc for Tor users, probably so they could leave the "normal" url in place. I think Facebook even had a stronger argument, which is that they've put a lot of comms training energy into teaching users that it must say "https://www.facebook.com/" and nothing else will do -- so it makes sense that they don't want to say "and that other one's ok too".

So here's the idea: make our browser recognize when it's using an onion alt-svc, and have it do the onion indicators in the urlbar like it does if there's a real onion address in the urlbar. Maybe also do something more obvious, to teach users that they're using an onion address right now -- all without messing with the "original brand" url that users still want to see.

(Calling @duncan's and @nah's names as the UX people.)