Merge branch 'transcription-null-zone' into 'master'

arkindex/documents/fixtures/data.json

arkindex/documents/management/commands/build_fixtures.py

.../documents/migrations/0014_nullable_transcription_zone.py

arkindex/documents/models.py

arkindex/documents/serializers/iiif/annotations.py

arkindex/documents/tests/test_annotation_list.py

arkindex/documents/tests/test_ml_results.py

arkindex/images/importer.py