Create a system note for new design discussions

What does this MR do?

This MR generates a new system note when a discussion is started on a design. Comment replies within the discussion do not generate system notes.

Issue: #11851 (closed).

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