Expose design images to front-end

What does this MR do?

This MR provides a new controller to expose the new Design Management design image files. The path to display a design image is now provided to clients through GraphQL's DesignType.

The controller takes a Design id, and optionally a ref. When given a ref the controller can be made to display a previous version of the design (the ref corresponding to the sha property of the design version). When no ref is passed the controller will return the latest design version.

The front-end Vue.js code has been updated to display the design images (previously placeholder images were displayed).

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 #11367 (closed)