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/DocumentPreview/DocumentPreview.js

@@ -181,7 +181,6 @@ const DocumentLink = styled(Link)`
   &:active,
   &:focus {
     background: ${(props) => props.theme.listItemHoverBackground};
-    outline: none;
 
     ${SecondaryActions} {
       opacity: 1;