Improve JSON-LD conformation to BioSchemas profile

[bencomp]

This addresses some of the suggestions I made in #481. I don't think I can address all the points (and I would like to discuss some of them before they are worked on), but I can remove the duplicate URLs if that is okay.

This currently makes the following changes:

• use LearningResource as type
• expand dct:conformsTo to full URI
• use (still hardcoded) Active as creativeWorkStatus (taken from https://bioschemas.org/profiles/TrainingMaterial/1.0-RELEASE)