Resolve "Design view: Download single issue design image"

What does this MR do?

This MR adds a button to single design view to allow downloading a design image.

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](https://docs.gitlab.com/ee/install/requirements.html#supported-web-browsers

Related issue: #34377 (closed)