Replace WorkerVersion.model_usage boolean with tri-state enum

arkindex/documents/fixtures/data.json

arkindex/documents/management/commands/bootstrap.py

arkindex/documents/management/commands/build_fixtures.py

arkindex/process/builder.py

arkindex/process/migrations/0001_initial.py

...rocess/migrations/0023_alter_workerversion_model_usage.py

arkindex/process/models.py

arkindex/process/serializers/imports.py

arkindex/process/serializers/training.py

arkindex/process/serializers/worker_runs.py

arkindex/process/serializers/workers.py

arkindex/process/tests/test_create_training_process.py

arkindex/process/tests/test_docker_worker_version.py

arkindex/process/tests/test_user_workerruns.py

arkindex/process/tests/test_workerruns.py

arkindex/process/tests/test_workers.py

arkindex/project/settings.py