Use turbolinks:load event listener

We use turbolinks so the DOMContentLoaded is not enough

app/javascript/burger_menu.js

@@ -1,4 +1,4 @@
-document.addEventListener('DOMContentLoaded', () => {
+document.addEventListener("turbolinks:load", function () {
   const $navbarBurgers = document.querySelectorAll('.navbar-burger');
   // Check if there are any navbar burgers
   if ($navbarBurgers.length > 0) {

app/javascript/demo.js

@@ -1,4 +1,4 @@
-document.addEventListener('DOMContentLoaded', function() {
+document.addEventListener("turbolinks:load", function () {
   var demoForm = document.getElementById('demo-form');
   if (!demoForm) {
     return;