Added Service Worker to Cache Assets for Offline

Added a service worker script to cache assets on local storage. In testing on desktop it worked. Need to test further on mobile.

index.html

@@ -186,7 +186,7 @@
         </div>
 
         <!-- Checks is there is a service worker and adds or updates -->
-        <!-- <script>
+        <script>
             if ('serviceWorker' in navigator) {
               window.addEventListener('load', () => {
                 navigator.serviceWorker.register('sw.js')
@@ -198,7 +198,7 @@
                 });
               });
             }
-        </script> -->
+        </script>
         <!-- <script src="js/vendor/modernizr-3.5.0.min.js"></script> -->
         <script>window.jQuery || document.write('<script src="js/vendor/jquery-3.2.1.min.js"><\/script>')</script>