Wait for the DOM to be ready before hiding the notification indicator.

Although the NotificationMenu is constructed on load, the DOM may not yet be
ready, meaning that the observer triggered by the .hide call cannot find any
other indicators to hide. Wait for the DOM to be ready to hide the notification
indicator.

Closes #14513

sao/src/notification.js

@@ -29,7 +29,11 @@
                     attributes: true,
                     attributeFilter: ['style'],
                 });
-            this.indicator.hide();
+            jQuery(document).ready(() => {
+                // Wait for the DOM to be ready to let the observer find the
+                // other indicators
+                this.indicator.hide();
+            });
         }
 
         fill() {