Add referrer policy setup check

Fixes #9122

Based on https://www.w3.org/TR/referrer-policy/ and
https://scotthelme.co.uk/a-new-security-header-referrer-policy/



Setting a sane Referrer-Policy will tell the browser if/when to send
referrer headers when accessing a link from Nextcloud. When configured
properly this results in less tracking and less leaking of (possibly)
sensitive urls

* Fix tests

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>