Export properties which don't have mappings

#781972: Use RestWS for non-RDFa serializations and content negotiation in Drupal 7 introduced a hidden feature which exports properties using a local site vocabulary namespace if no mapping exist. This feature is disabled by default. Let's add an option in the UI for enabling it and expose the site vocabulary namespace.

Related: #1053928: Port site vocabulary functionality to Drupal 7 which might be considered a requirement if one wants the site vocabulary namespace to dereference.