Set Referrer-Policy also in addSecurityHeaders()

Fix: #12689
Signed-off-by: Peter Kraume <peter.kraume@gmx.de>

lib/private/legacy/response.php

@@ -104,6 +104,7 @@ public static function addSecurityHeaders() {
 			header('X-Robots-Tag: none'); // https://developers.google.com/webmasters/control-crawl-index/docs/robots_meta_tag
 			header('X-Download-Options: noopen'); // https://msdn.microsoft.com/en-us/library/jj542450(v=vs.85).aspx
 			header('X-Permitted-Cross-Domain-Policies: none'); // https://www.adobe.com/devnet/adobe-media-server/articles/cross-domain-xml-for-streaming.html
+			header('Referrer-Policy: no-referrer'); // https://www.w3.org/TR/referrer-policy/
 		}
 	}