Convert HTML metadatas as markdown

.isort.cfg

...ents/migrations/0030_convert_html_metadata_as_markdown.py

arkindex/documents/models.py

arkindex/documents/serializers/iiif/manifests.py

arkindex/documents/serializers/light.py

arkindex/documents/tests/test_manifest.py

arkindex/documents/tests/test_metadata.py

requirements.txt