[Kickoff] Revamp LeaveButtons item

* Simplify to be a `Repeater` in a `ColumnLayout`
* Popuplate context menu with an `Instantiator` with `MenuItem` as delegate
* Elide power buttons if necessary (previously they would just be cut off/fall off the view)
* Position context menu below/above the leave button
* Make leave button "pressed" while the menu is open


(cherry picked from commit 19b1507b)