Move Design Management models to FOSS

What does this MR do?

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

This MR focuses on moving:

• the core Design Management models (ee/app/models/design_management).
• the bare minimum other model code necessary to support the core Design Management models.
• model specs and factories.

There are a few extra non-core Design Management model pieces that are included in this MR:

• an uploader that is mounted in DesignManagement::Action
• a service that is invoked in DesignManagement::Design#user_notes_count_service

There is some remaining Design Management-related code in other models that will be moved later (in order to keep this MR as small as possible!).

There are a few tests that are skip'd when run in FOSS. These are tests that rely on other parts of the application, like services or non-core Design Management model code referred to above, being ported to FOSS, and they are not included in this MR. I chose to use skip rather than to keep fragments in EE-specific specs, because the methods themselves have moved to FOSS, however they do not perform as they do in EE yet. The skips are being tracked in #212566 (comment 327724283).

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