fix: Focus accessibility (#1536)

* fix: Focus ring appearing on click
fix: Focus ring not appearing on sidebar links
add: focus-visible polyfill

* fix: More visible outlines on keyboard focus
fix: Header block should be a button semantically

app/components/NudeButton.js

@@ -1,5 +1,4 @@
 // @flow
-import { lighten } from "polished";
 import * as React from "react";
 import styled from "styled-components";
 
@@ -11,13 +10,6 @@ const Button = styled.button`
   line-height: 0;
   border: 0;
   padding: 0;
-
-  &:focus {
-    transition-duration: 0.05s;
-    box-shadow: ${(props) => lighten(0.4, props.theme.buttonBackground)} 0px 0px
-      0px 3px;
-    outline: none;
-  }
 `;
 
 export default React.forwardRef<any, typeof Button>((props, ref) => (