feat: Resizable sidebar (#1827)

* wip: First round on sidebar resizing

* feat: Saving setting, animation

* all requirements, refactoring needed

* lint

* refactor useResize

* some mobile improvements

* fix

* refactor

app/components/SkipNavContent.js

@@ -0,0 +1,8 @@
+// @flow
+import * as React from "react";
+
+export const id = "skip-nav";
+
+export default function SkipNavContent() {
+  return <div id={id} />;
+}