Avoid refetching image urls for designs

What does this MR do?

This MR allows us not to refetch a design image every single time we open a design. We want to update pins and discussions but the image should remain cached. In order to do this, we pass a flag to design query: if we fetched the design once, we include only discussions to the query.

I've also flattened a discussions structure a bit: we don't need edges now so I went for nodes directly

Screenshots

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

Issue #213940 (closed)