Switch javascript to ES6 syntax

website/layouts/index.html

@@ -261,10 +261,10 @@
     </div>
   </div>
 <script>
-  var modal = document.getElementById("ml-modal");
-  var modal_btn = document.getElementById("ml-modal-open");
-  var close_btn = document.getElementsByClassName("close-btn")[0];
-  var subscribe_btn = document.getElementsByClassName("subscribe-btn")[0];
+  const modal = document.getElementById("ml-modal");
+  const modal_btn = document.getElementById("ml-modal-open");
+  const close_btn = document.getElementsByClassName("close-btn")[0];
+  const subscribe_btn = document.getElementsByClassName("subscribe-btn")[0];
 
   modal_btn.onclick = function() {
     modal.style.display = "block";