Allow private comments on Service Desk issues
Closed as a duplicate - actual epic is https://gitlab.com/groups/gitlab-org/-/epics/2697
### Problem to solve
In a Service Desk situation, it is sometimes desirable to discuss some matter privately within the support team and internal participants to an issue, without creating a separate issue, which is cumbersome because it breaks the flow and timeline of the support issue. Having a means of making public and private comments is extremely useful in those cases.
### Intended users
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
* [Sidney (Systems Administrator)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sidney-systems-administrator)
* [Sam (Security Analyst)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sam-security-analyst)
* Support Engineer
> As a software developer, I would like to collaborate with the support team directly on a Service Desk issue, without providing visibility to the end-user.
> As a security analyst, I would like to be able to offer security / vulnerability assessments within issues without worry of providing potentially sensitive information to the end-user.
> As a support engineer, I need the ability to collaborate on Service Desk issues internally with the above personas to formulate an appropriate response to the end-user.
### Proposal
Add a checkbox under the comment box textarea to allow the user to mark the comment as `Private`.
For MVC, if a comment is marked `Private`, any and all replies will inherit this setting, with no option to mark it `Public`. Hence, a `Private` thread will always remain `Private`.
All comments added via email reply will be considered `Public` as they are now.
Designs available [here](https://gitlab.com/gitlab-org/gitlab/issues/2459/designs/private-comment.png) (in the design tab).
### Documentation
* Update documentation to describe use of `Public` vs. `Private` comment.
* Ensure it is clear who has visibility to `Private` comments (Service Desk users)
* Clarify that email responses will be `public` comments.
epic