Allow context trimmer to resolve URL references

[spikelynch]

I tried adding the GeoJSON-LD context to the @context (for a data publication with GeoJSON spatial coverage) but the jsonld_helper trim_context function is stripping them back out again.

Getting it to resolve URL references would be a start, but ideally we'd find a way to use the json-ld library to do this so we don't have to.

I'm hacking around it in datacrate (https://code.research.uts.edu.au/eresearch/datacrate) for now