Extend Design view sidebar with issue link and a list of participants

What does this MR do?

The design view needs more context about the issue and who is involved. When writing a comment on a design, it would be nice to know who else is involved in the issue so I can cc them, and get at least the title of the issue and more info surrounding the design I'm uploading.

This MR adds an issue link and a list of issue participants to the design sidebar. When a new discussion/comment is created, the participant list is updated.

Screenshots

Before	After

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

Issue: #13490 (closed)