GraphQL mutations for managing Notes (EE)

What does this MR do?

This is the EE branch for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30210.

It contains the CE code, plus exposes diff_refs in DesignType.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers