[UX] Refine permissions for Field UI

I ran into a problem on a Drupal 7 site yesterday where the permissions for "Manage fields" and "Manage display" were not handled consistently across different entity types - creating a very confusing experience for users.

My use case: I wasn't able to determine which permission I needed to grant to allow a role access to manage fields on files entity.

Fortunately, I stumbled across this excellent module which lead me to this Drupal core issue where they are also working on the problem.

I would love to see the same fix for Backdrop.