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/components/Sidebar/Main.js

@@ -75,16 +75,17 @@ class MainSidebar extends React.Component<Props> {
 
     return (
       <Sidebar>
-        <AccountMenu
-          label={
+        <AccountMenu>
+          {(props) => (
             <HeaderBlock
+              {...props}
               subheading={user.name}
               teamName={team.name}
               logoUrl={team.avatarUrl}
               showDisclosure
             />
-          }
-        />
+          )}
+        </AccountMenu>
         <Flex auto column>
           <Scrollable shadow>
             <Section>