Move Design Management-related model code to FOSS

What does this MR do?

This change is part of #212566 (closed) to move all Design Management code to FOSS.

This MR moves model code that references parts of Design Management. The "core" Design Management model code has been moved in !30015 (merged).

A changelog entry, and any documentation changes, will be added in the final MR.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done