Merge branch 'nuke-metadata-index' into 'master'

arkindex/documents/fixtures/data.json

arkindex/documents/migrations/0044_remove_metadata_index.py

arkindex/documents/models.py

arkindex/documents/serializers/light.py

arkindex/documents/tests/tasks/test_export.py

arkindex/documents/tests/test_children_elements.py

arkindex/documents/tests/test_corpus_elements.py

arkindex/documents/tests/test_metadata.py

arkindex/documents/tests/test_parents_elements.py

arkindex/sql_validation/indexer_prefetch.sql