Show DBpedia classes whose only superclass is an external one on mappings server

Currently, DBpedia classes which only have external superclasses are not shown on the ontology classes overview page (http://mappings.dbpedia.org/server/ontology/classes/), e.g., the `dbp:Place` class.
This push request fixes the issue by only considering classes which are actually shown on the overview page when grouping by base class. If this leads to an empty set of base classes, `owl:Thing` serves as a fallback.