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/OverflowMenuButton.js

@@ -0,0 +1,21 @@
+// @flow
+import { MoreIcon } from "outline-icons";
+import * as React from "react";
+import { MenuButton } from "reakit/Menu";
+import NudeButton from "components/NudeButton";
+
+export default function OverflowMenuButton({
+  iconColor,
+  className,
+  ...rest
+}: any) {
+  return (
+    <MenuButton {...rest}>
+      {(props) => (
+        <NudeButton className={className} {...props}>
+          <MoreIcon color={iconColor} />
+        </NudeButton>
+      )}
+    </MenuButton>
+  );
+}