Add confidential issue notice in comment box.
What does this MR do?
Adds a warning about confidential issues to the comment field when the issue is confidential.
Are there points in the code the reviewer needs to double check?
I don't think so.
Why was this MR needed?
It wasn't clear to users that they were commenting on a confidential issue, this makes it more explicit.
What are the relevant issue numbers?
Fixes #15288 (closed)
Screenshots (if relevant)
This is what it looks like at various screen sizes: