[EE] Reduce diff with CE in spec/fixtures/api/schemas

CE backport can be found at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26048.

What does this MR do?

This extracts EE-specific code from spec/fixtures/api/schemas.

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/9506.

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
• Security reports checked/validated by reviewer