Prevent context menu on option long tap in iOS.

If the option contains an image, iOS would pop up a menu with image
related options. We fixed a similar problem in Android where we prevent
default action on contextmenu events, but that does not seem to work on
iOS, which requires a special CSS property.

drag_and_drop_v2/public/css/drag_and_drop.css

@@ -123,6 +123,7 @@
     outline-style: none;
     /* Some versions of the drag and drop library try to fiddle with this */
     z-index: 10 !important;
+    -webkit-touch-callout: none;
 }
 
 @media screen and (max-width: 480px) {