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/components/GroupMemberListItem.js

@@ -1,28 +1,21 @@
 // @flow
 import * as React from "react";
-import GroupMembership from "models/GroupMembership";
 import User from "models/User";
 import Avatar from "components/Avatar";
 import Badge from "components/Badge";
 import Button from "components/Button";
-import { DropdownMenu, DropdownMenuItem } from "components/DropdownMenu";
 import Flex from "components/Flex";
 import ListItem from "components/List/Item";
 import Time from "components/Time";
+import GroupMemberMenu from "menus/GroupMemberMenu";
 
-type Props = {
+type Props = {|
   user: User,
-  groupMembership?: ?GroupMembership,
   onAdd?: () => Promise<void>,
   onRemove?: () => Promise<void>,
-};
+|};
 
-const GroupMemberListItem = ({
-  user,
-  groupMembership,
-  onRemove,
-  onAdd,
-}: Props) => {
+const GroupMemberListItem = ({ user, onRemove, onAdd }: Props) => {
   return (
     <ListItem
       title={user.name}
@@ -42,11 +35,7 @@ const GroupMemberListItem = ({
       image={<Avatar src={user.avatarUrl} size={40} />}
       actions={
         <Flex align="center">
-          {onRemove && (
-            <DropdownMenu>
-              <DropdownMenuItem onClick={onRemove}>Remove</DropdownMenuItem>
-            </DropdownMenu>
-          )}
+          {onRemove && <GroupMemberMenu onRemove={onRemove} />}
           {onAdd && (
             <Button onClick={onAdd} neutral>
               Add