Adds SemanticData from subobjects to the RDF export fixing https://bugzilla.wikimedia.org/show_bug.cgi?id=48708

Note: This is a partial integration of the patch https://gerrit.wikimedia.org/r/#/c/70047/4
Credits to Dbeeson

Code changes in short:
The Export code for RDF now understands that SemanticData has recursive information about subobjects and tries to iterate over them to add to the RDF export.