Fixes RDF export of subobjects

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.
There is no special meaning to '#-' other than as an terminator to the name of a subobject when that object is retrieved in an #ask query. I wanted to use a terminator because it is otherwise unclear whether the referenced object is the subobject or the page on which the subobject is declared. I can provide an example if you want.