EE: Refactor how a few ActiveRecord enums are defined

What does this MR do?

This is the EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23024.

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ee/issues/8244
• https://gitlab.com/gitlab-org/gitlab-ee/issues/8241

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder