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/Separator.js

@@ -0,0 +1,16 @@
+// @flow
+import * as React from "react";
+import { MenuSeparator } from "reakit/Menu";
+import styled from "styled-components";
+
+export default function Separator(rest: {}) {
+  return (
+    <MenuSeparator {...rest}>
+      {(props) => <HorizontalRule {...props} />}
+    </MenuSeparator>
+  );
+}
+
+const HorizontalRule = styled.hr`
+  margin: 0.5em 12px;
+`;