Close EventSource before unloading the page (#11539)

Should eliminate a error in the Firefox console regarding the connection
being interrupted while the page was loading.

web_src/js/features/notification.js

@@ -52,6 +52,9 @@ export function initNotificationCount() {
       source.close();
       window.location.href = AppSubUrl;
     });
+    window.addEventListener('beforeunload', () => {
+      source.close();
+    });
     return;
   }