fix: Keyboard accessible context menus (#1768)

- Makes menus fully accessible and keyboard driven
- Currently adds 2.8% to initial bundle size due to the inclusion of Reakit and its dependency, popperjs.
- Converts all menus to functional components
- Remove old custom menu system
- Various layout and flow improvements around the menus

closes #1766

app/scenes/GroupMembers/AddPeopleToGroup.js

@@ -112,7 +112,6 @@ class AddPeopleToGroup extends React.Component<Props> {
               key={item.id}
               user={item}
               onAdd={() => this.handleAddUser(item)}
-              canEdit
             />
           )}
         />