Make our SPARQL queries compatible with non-Blazegraph endpoints

[Abbe98]

Description

The Wikidata endpoint is struggling more and more and even if one uses our shared SPARQL cache, one isn't unlikely to have ones build interrupted by timeouts. If the status quo persist we will need to move off WDQS at one point.

In the meanwhile I think we should aim to move our SPARQL queries away from WDQS/Blazegraph specific SERVICEs and syntax. For us I think this mainly means replacing the wikibase:label service. Such a move would make it not only easier to move to another endpoint one day but it will also make it easier to experiment with WDQS alternatives.

[Ainali]

Should we also make sure we always have the right prefixes added?

[Abbe98]

No I don't think that should be a priority as most SPARQL backends support per-defined prefixes.