Move focus back to button efter user tab out of last link in mobile menu.

assets/js/mobile.js

@@ -45,4 +45,8 @@
     });
   });
 
+  // Move focus back to button efter user tab out of last link.
+  const lastlink = [].slice.call(links).pop();
+  lastlink.addEventListener('blur', () => toggle.focus());
+
 })();