Support WorkerRun on Classification

1. add worker_run_id FK on Classification (nullable, blank)
2. make it available in the admin details of an Classification, using raw_id_field
3. expose it in CreateClassification endpoint (required for UFCN)
4. bonus: expose it in CreateClassifications endpoint

Rules for endpoints:

• we only support worker_version_id XOR worker_run_id, raise a 400 if both are supplied
• the endpoint must load the WorkerRun from its ID to check its existence
• and use the related WorkerVersion as worker_version_id on the resulting classifications (so we end up with both worker_version & worker_run being set on the Classification)