Add JSON schema validation to gbl-1_to_aardvark

[srappel]

Confirm the JSON files are valid against our schema.

[srappel]

gbl_to_aardvark.py itself does not validate against the JSON schema. It mainly decides which repos need conversion and then shells out to convert.py.

In the current app code, the step that does schema validation is DCAT_Harvester.py. It loads the schema URL from config/opendataharvest.yaml (line 51), fetches it, and calls jsonschema.validate(...); you can see that around DCAT_Harvester.py (line 502) and where it validates records before writing around DCAT_Harvester.py (line 766).

harvest_dcat: yes, schema validation happens
gbl_to_aardvark / convert.py: no JSON Schema validation
normalize.py: no JSON Schema validation