Set Referrer-Policy to no-referrer

Fandom sends a fake 404 to media if there's a Referer header that has an origin
that's not Fandom. However, we can choose not to send the header by setting
Referrer-Policy. See also:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referrer-Policy

src/page-category.rkt

@@ -113,6 +113,7 @@
       (xexp->html body))
     (response/output
      #:code 200
+     #:headers (list referrer-policy)
      (λ (out)
        (write-html body out))))))
 (module+ test