Merge branch 'remove-sources' into 'master'

arkindex/documents/api/entities.py

arkindex/documents/api/ml.py

...s/migrations/0020_remove_source_xor_version_constraint.py

arkindex/documents/models.py

arkindex/documents/serializers/elements.py

arkindex/documents/serializers/entities.py

arkindex/documents/serializers/ml.py

arkindex/documents/tests/test_bulk_classification.py

arkindex/documents/tests/test_bulk_element_transcriptions.py

arkindex/documents/tests/test_create_elements.py

arkindex/documents/tests/test_create_transcriptions.py

arkindex/documents/tests/test_entities_api.py

arkindex/documents/tests/test_moderation.py

arkindex/project/serializer_fields.py

arkindex/project/tests/test_datasource_slug_field.py