enhanced demo.js alert with a 15min timeout

dist/js/demo.js

@@ -10,8 +10,9 @@
 (function ($) {
   'use strict'
 
-  setTimeout(() => {
-    if (window.___browserSync___ === undefined) {
+  setTimeout(function () {
+    if (window.___browserSync___ === undefined && Number(localStorage.getItem('AdminLTE:Demo:MessageShowed')) < Date.now()) {
+      localStorage.setItem('AdminLTE:Demo:MessageShowed', (Date.now()) + (15 * 60 * 1000))
       // eslint-disable-next-line no-alert
       alert('You load AdminLTE\'s "demo.js", \nthis file is only created for testing purposes!')
     }