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/Sidebar/components/HeaderBlock.js

@@ -57,10 +57,16 @@ const TeamName = styled.div`
   font-size: 16px;
 `;
 
-const Header = styled(Flex)`
+const Header = styled.button`
+  display: flex;
+  align-items: center;
   flex-shrink: 0;
   padding: 16px 24px;
   position: relative;
+  background: none;
+  line-height: inherit;
+  border: 0;
+  margin: 0;
   cursor: pointer;
   width: 100%;