On a v2 onion page, Onion-Location with a v3 onion should count as "onion available"

If I load a v2 onion service, and it gives me an Onion-Location header listing a v3 onion service, then we should show the "onion available" button to the user, auto switch to it if their Tor Browser is configured to auto switch when onion is available, etc.

This way there can be an authenticated way to update people from a site's legacy v2 address to their new v3 address.

Originally suggested by Alec on https://lists.torproject.org/pipermail/tor-dev/2020-June/014372.html