Adopt new download_urls array property

[giohappy]

Since GeoNode/geonode#11421 the resource API endpoints return a new array property for the download URL:

 "download_urls": [
   {
      "URL": .... , 
      "ajax_safe": true|false, 
      "default":true|false
   },
]

It's array in view of supporting multiple download URLs. At the moment only one value is returned, with the default property set to true.

The relevant addition is the ajax_safe property. This value tells the client if the download URL can be called with AJAX or if an href should be used instead. This information is important for specific downloads that do not use the GeoNode backend but use instead external URLs that might not support CORS.

[giohappy]

moved to GeoNode#1587