Set Cache-Control to 60s on failed fetch

This way we can permit a fast retry while keeping some control on DoS attacks

render_relay.go

@@ -74,6 +74,8 @@ func renderRelayPage(w http.ResponseWriter, r *http.Request) {
 
 	if len(renderableLastNotes) != 0 {
 		w.Header().Set("Cache-Control", "max-age=604800")
+	} else {
+		w.Header().Set("Cache-Control", "max-age=60")
 	}
 
 	if err := tmpl.ExecuteTemplate(w, templateMapping[typ], params); err != nil {