Support ontologies from a SPARQL Endpoint

Ontologies could be published only via a SPARQL endpoint, i.e., without a LD interface. As user, I want to specify a SPARQL endpoint where I have all my ontologies, so that I could use them for completion suggestions and quick documentation.

• already registered SPARQL endpoints could be used, instead of maintaining a separate registry

• instead of issuing SPARQL queries for every completion suggestion request, it would be better to index ontologies locally

• it has to be a project-specific configuration because SPARQL endpoints are stored in project-specific files