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 

## Screenshots (if relevant)

This is what it looks like at various screen sizes:

![Screen_Shot_2016-06-03_at_2.56.54_PM](/uploads/eb7f318bc19101f8514d7a4f10a77d73/Screen_Shot_2016-06-03_at_2.56.54_PM.png)

![Screen_Shot_2016-06-03_at_2.58.51_PM](/uploads/13674c3f1227b0d566b1d2aafc6712ed/Screen_Shot_2016-06-03_at_2.58.51_PM.png)

![Screen_Shot_2016-06-03_at_2.58.37_PM](/uploads/14ad419ed3ed86b7fbc080152e4030fa/Screen_Shot_2016-06-03_at_2.58.37_PM.png)

cc: @jschatz1 @dzaporozhets 

See merge request !4473