Add rdf_subject to factories WebResource instances

See: https://github.com/dpla/KriKri/commit/727239a3de2b444d57aeb51c9f78a4fbaf78b878#diff-16ca0a4bf0c02db9d9f7b294c486976bR33
This should be a link to the asset itself, not the object in the DPLA frontend. Use something like `http://images.nypl.org/index.php?id=1582254&t=w`?