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/ContextMenu/Header.js

@@ -0,0 +1,13 @@
+// @flow
+import styled from "styled-components";
+
+const Header = styled.h3`
+  font-size: 11px;
+  font-weight: 600;
+  text-transform: uppercase;
+  color: ${(props) => props.theme.sidebarText};
+  letter-spacing: 0.04em;
+  margin: 1em 12px 0.5em;
+`;
+
+export default Header;