Move abilities by subject class to a dedicated method (EE)

What does this MR do?

Twin of gitlab-org/gitlab-ce!5668.

It moves the case that returns abilities depending on the subject class to a dedicated method.

This should avoid lame conflicts when CE to EE since the EE version assign the result of the case to a variable.

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#20621.

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together