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/DropdownMenu/DropdownMenuItem.js

@@ -80,7 +80,6 @@ const MenuItem = styled.a`
   &:focus {
     color: ${props.theme.white};
     background: ${props.theme.primary};
-    outline: none;
   }
   `};
 `;