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
Implement custom notification level options
![Screen_Shot_2016-06-17_at_15.31.43](/uploads/3fc47d2f461b3e8b67bb8acaa304cf99/Screen_Shot_2016-06-17_at_15.31.43.png)
![Screenshot_from_2016-06-15_10-52-27](/uploads/88dbdd21d97e80ee772fe08fa0c9b393/Screenshot_from_2016-06-15_10-52-27.png)
part of #12758
See merge request !4389