Make the preview enricher add viewUrl and downloadUrl

After implementing ELEMENTS-1572 to distinguish views from downloads, the elements are using 2 new properties "viewUrl" and "downloadUrl".

Some elements (maybe only nuxeo-document) are extending the response to add 2 new properties to the Document object from the "preview" context parameter. But when the documents are returned by another element (like nuxeo-page-provider), the "preview" context parameter is not extended and the Nuxeo elements will fail because they cannot find "viewUrl" or "downloadUrl".

A simple solution to handle this problem is to make the Preview enricher compute viewUrl and downloadUrl and put them to the JSON response