installation/pleroma.nginx: Add Content-Security-Policy

Closes: https://git.pleroma.social/pleroma/pleroma/issues/266

installation/pleroma.nginx

@@ -77,6 +77,7 @@ server {
         add_header X-Content-Type-Options nosniff;
         add_header Referrer-Policy same-origin;
         add_header X-Download-Options noopen;
+        add_header Content-Security-Policy "default-src 'none'; base-uri 'self'; form-action 'self'; img-src 'self' data: https:; media-src 'self' https:; style-src 'self' 'unsafe-inline'; font-src 'self'; script-src 'self'; connect-src 'self' wss://example.tld; upgrade-insecure-requests;";
         
         # Uncomment this only after you get HTTPS working.
         # add_header Strict-Transport-Security "max-age=31536000; includeSubDomains";