Modify `DiffNote` to reuse it for Designs

What does this MR do?

This modifies the DiffNote model so we can create DiffNotes for DesignManagement::Designs.

It also includes some specs illustrating how the Notes::CreateService will need to be used from GraphQL for designs

This is part of the work for: https://gitlab.com/gitlab-org/gitlab-ee/issues/9489

CE changes in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29023

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry Not user visible
• [-] 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