Render embedded remote documents with the embed template

[giohappy]

At the moment the embed URL for remote documents returns a redirection to the remote URL. This automatically makes the browser show the preview if the remote document doesn't have the content-disposition set, otherwise the download of the document is automatically triggered instead of showing the preview.

The client has implemented a new mechanism in GeoNode/geonode-mapstore-client#1642 to:

• test if the remote resource has CORS enabled -> the document is downloadad and the Blob for the iframe is created
• if CORS is disabled it tried to get it through the GeoNode proxy -> if the proxy has the remote host set in the PROXY_ALLOWED_HOSTS it retrieves the remote document -> the document is downloadad and the Blob for the iframe is created
• if all the above steps fail the client doesn't show the preview

With this support from the client side we can make the document embed template render also for remote documents, instead of redirecting to the remote URL.