Add the Onion-Location header to the Tails website

I think that adding a Onion-Location header to the Tails website would cause no harm (eg. risk confusing users or slowing down the connection) assuming that Tor Browser will not redirect automatically to the Onion service (see tails#17936 (comment 157254) and the header's spec proposal).

Benefits:

• Gives users the possibility of not using an exit relay to access the Tails website.
• Helps strengthen the Tor ecosystem.
• Possibly allows a smoother transition to automatically redirecting in the future if we decide to do so.

Notes:

• This does not implement the Alt-Svc header (comment, example), as this would need a bit more research from my side.

I'd like to hear an OK from @intrigeri and @sajolida (because they expressed concerns) and from @groente (because he was involved in making the website reachable via Onion service in the first place).