Add ontology support

[PGijsbers]

With the upgrade to the new dataset image (#236), there is now ontology data in the database.
To have feature parity with PHP, we need to add it here too.
Places where ontologies are used:

• feature descriptions
• POST /data/features

See also https://github.com/openml/OpenML/blob/b1265bda1b52780a6f10a7d239e3f54914517c2f/openml_OS/views/pages/api_new/v1/xsd/openml.data.features.xsd#L2