Hides merge request section in edit project when disabled

## What does this MR do?

Correctly hides the merge request section in the project edit page when disabled. It used to work before the feature visibility part was changed to dropdown - so added a test to prevent in future! 👍 

## Screenshots (if relevant)

![hide](/uploads/eb72ef87bad19c2716472a0c1031fd32/hide.gif)

See merge request !6175