Return Todos for Designs via the REST API

What does this MR do?

This MR enables Todos for Designs to be returned via the REST API.

This feature had previously been intentionally disabled in !15129 (merged) (see /ee/lib/ee/api/todos.rb and discussion #13494 (comment 203827901)).

Issue #13543 (closed)

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