Export design management data with project export

What does this MR do?

Issue https://gitlab.com/gitlab-org/gitlab-ee/issues/11090.

This MR allows Design Management data to be included in the project import/export.

A separate MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14700 contains changes to support the project's design repository. This MR will remain a WIP: until !14700 (merged) is merged into this MR.

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

Closes #11090 (closed)