Use jQuery objects in ref dropdown
## What does this MR do?
Keeps the behaviour the same as the ref dropdown, however it now uses jQuery objects to handle the HTML creation instead of passing strings around. It pretty much worked out the box so not much was changed in the dropdowns themselves. Will be easy to transfer this over to other dropdown menus as well.
cc. @jschatz1 @rspeicher @DouweM
Do we want this to go into a patch release?
See merge request !4850
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)

See merge request !4615
Refs dropdown is now loaded async
## What does this MR do?
The refs dropdown is loaded async so not to block the page.
## What are the relevant issue numbers?
Part of #18202
## Screenshots (if relevant)

See merge request !4508
Implement custom notification level options


part of #12758
See merge request !4389
* visiting project will create notification setting if missing
* change notification setting per project even without membership
* use notification settings instead of membership on profile page
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
* more focus for description and stars/forks
* rename activity -> to project tab
* put activity under tab: prepare place for README tab
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>