Fixed issue with notification dropdown not updating active

## What does this MR do?

This fixes an issue where the notification dropdown wouldn't correctly update the selected notification type in the dropdown. I've also added a spinner to show the action is happening & removed the flash message for success.

## What are the relevant issue numbers?

Closes #18480 

## Screenshots (if relevant)

![notifications](/uploads/5ed5055d180c12450cea77543ae5f44e/notifications.gif)

See merge request !4615