Resolve "Add `/confidential` quick action for issues"

What does this MR do?

Adds a /confidential quick command.

Are there points in the code the reviewer needs to double check?

No

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered

What are the relevant issue numbers?

Closes #47145 (closed)