Add `confidential` quick action to epics

Summary

Allow /confidential quick action on epics.

Docs Support

For issues requiring an update to GitLab docs, please note that this is part of the definition of done for this issue. Please create a separate MR for the docs update related to the MR for the actual change to ensure code reviews are not blocked by TW reviews.

• This issue requires an update to GitLab docs
• This issue does not require an update to GitLab docs