R1:9658249ac5a0

Add "Referrer-Policy: no-referrer" to standard HTTP headers

Summary:
Ref T4340. Some browsers respect this header and referrers are a plague upon the earth.

Also, upgrade "never" to the more modern value "no-referrer".

Test Plan:
In Safari, Firefox and Chrome, disabled `rel="noreferrer"` on links and generated a normal link to an external site. Then clicked it and checked if a referrer was sent.

- Safari respects meta only, but "no-referrer" is fine.
- Firefox respects both (either the header or…