Set Referrer-Policy to no-referrer for media (#7009)

changelog.d/7009.feature

+Set `Referrer-Policy` header to `no-referrer` on media downloads.

synapse/rest/media/v1/download_resource.py

@@ -50,6 +50,9 @@ class DownloadResource(DirectServeResource):
             b" media-src 'self';"
             b" object-src 'self';",
         )
+        request.setHeader(
+            b"Referrer-Policy", b"no-referrer",
+        )
         server_name, media_id, name = parse_media_id(request)
         if server_name == self.server_name:
             await self.media_repo.get_local_media(request, media_id, name)