Update cookie notice usage

Only show if not shown within last 30 days or cookie showing user has 
accepted cookies doesn't exist

luhn-algorithm/index.html

@@ -30,6 +30,9 @@
         <!-- import jQuery -->
         <script src="/assets/jquery.min.js"></script>
 
+        <!-- import cookie JavaScript -->
+        <script src="/assets/cookies.js"></script>
+
         <!-- import Material Design components & icons -->
         <link href="/assets/material-components-web.min.css" rel="stylesheet">
         <link href="/assets/material_icons.css" rel="stylesheet">