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/Tab.js

@@ -1,5 +1,4 @@
 // @flow
-import { lighten } from "polished";
 import * as React from "react";
 import { NavLink } from "react-router-dom";
 import styled, { withTheme } from "styled-components";
@@ -23,13 +22,6 @@ const StyledNavLink = styled(NavLink)`
     border-bottom: 3px solid ${(props) => props.theme.divider};
     padding-bottom: 5px;
   }
-
-  &:focus {
-    outline: none;
-    border-bottom: 3px solid
-      ${(props) => lighten(0.4, props.theme.buttonBackground)};
-    padding-bottom: 5px;
-  }
 `;
 
 function Tab({ theme, ...rest }: Props) {