[#1899] Add allow-downloads to iframe sandbox attributes

Author: catarak

client/modules/IDE/components/PreviewFrame.jsx

@@ -23,7 +23,7 @@ function PreviewFrame({ fullView }) {
 
   const frameUrl = previewUrl;
   const sandboxAttributes = `allow-forms allow-modals allow-pointer-lock allow-popups 
-    allow-same-origin allow-scripts allow-top-navigation-by-user-activation`;
+    allow-same-origin allow-scripts allow-top-navigation-by-user-activation allow-downloads`;
   const allow =
     'accelerometer; autoplay; camera; encrypted-media; geolocation; gyroscope; microphone; magnetometer; midi; vr;';
   return (