Added icon for iOS web app

If you using Safari on a iOS device and save the page to home screen you will get the Grafana icon as icon

public/views/index.template.html

@@ -15,6 +15,7 @@
 
   <link rel="icon" type="image/png" href="public/img/fav32.png">
   <link rel="mask-icon" href="public/img/grafana_mask_icon.svg" color="#F05A28">
+  <link rel="apple-touch-icon" href="public/img/fav32.png">
   
 </head>