DEV: Add optional callback in notification dropdown action (#31257)

Modify the component UserNotificationDropdown to receive a function that
will be called after an user is ignored.

app/assets/javascripts/select-kit/addon/components/user-notifications-dropdown.js

@@ -66,6 +66,7 @@ export default class UserNotificationsDropdown extends DropdownSelectBox {
       model: {
         ignoredUsername: this.user.username,
         enableSelection: false,
+        onUserIgnored: (username) => this.onUserIgnored?.(username),
       },
     });
   }