Use const to declare service worker JS variable

Author: rxu

styles/prosilver/template/event/overall_footer_after.html

@@ -1,4 +1,4 @@
 <script>
-	var serviceWorkerUrl = '{{ U_WEBPUSH_WORKER_URL }}';
+	const serviceWorkerUrl = '{{ U_WEBPUSH_WORKER_URL }}';
 </script>
 {% INCLUDEJS('@phpbb_webpushnotifications/update_worker.js') %}