Move confidentiality from the sidebar to the header

Summary

Moving Confidentiality as an action to the header will align with other changes proposed in #373757 (closed), and with the overall strategy of using the actions menu to handle "meta" actions on an Issue separate from the attributes in the sidebar.

Considerations

• The meaning of "Confidentiality" may not be readily apparent. We currently rely on an interstitial to convey this meaning: "You are going to turn on confidentiality. Only project members with at least the Reporter role, the author, and assignees can view or be notified about this issue."
• The state of an item being not confidential would need to be inferred from the lack of a badge if in the menu

UX

1. Add a menu action titled "Turn [on|off] confidentiality"
2. Confidential badge immediately appears/disappears on change
3. Toast is shown "Confidentiality turned [on|off"