Add import endpoints for external ontologies

This MR adds three new endpoints for importing resources, classes and predicates from external ontologies. Each thing can either be imported by ontology id and URI/IRI or by ontology id and short form id. The label and the description are automatically fetched from the respective ontology provider and linked via statements to the newly created thing, as well as a SAME_AS statement, linking to the original source. For now, we support Wikidata (all thing types), Geonames (resources) and OLS (classes).

Remarks:

• The endpoints always return status 201 CREATED, even if the resource already existed in the graph. We might want to change this prior to merging
• We only allow importing things for a given ontology. However, it might make sense to have an import endpoint that is not tied to a known ontology provider, so arbitrary things can be imported.
• Created SAME_AS statements are not set to modifiable=false for now, as the modifiability functionality still requires some refactoring.