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/SkipNavLink.js

@@ -0,0 +1,34 @@
+// @flow
+import * as React from "react";
+import styled from "styled-components";
+import { id } from "components/SkipNavContent";
+
+export default function SkipNavLink() {
+  return <Anchor href={`#${id}`}>Skip navigation</Anchor>;
+}
+
+const Anchor = styled.a`
+  border: 0;
+  clip: rect(0 0 0 0);
+  height: 1px;
+  width: 1px;
+  margin: -1px;
+  padding: 0;
+  overflow: hidden;
+  position: absolute;
+  z-index: 1;
+
+  &:focus {
+    padding: 1rem;
+    position: fixed;
+    top: 12px;
+    left: 12px;
+    background: ${(props) => props.theme.background};
+    color: ${(props) => props.theme.text};
+    outline-color: ${(props) => props.theme.primary};
+    z-index: ${(props) => props.theme.depths.popover};
+    width: auto;
+    height: auto;
+    clip: auto;
+  }
+`;