Merge branch 'no-thumbnails' into 'master'

arkindex/documents/admin.py

arkindex/documents/api/elements.py

arkindex/documents/fixtures/data.json

...ndex/documents/migrations/0059_remove_corpus_thumbnail.py

arkindex/documents/models.py

arkindex/documents/serializers/elements.py

arkindex/documents/tests/test_corpus.py

arkindex/sql_validation/corpus_delete.sql

arkindex/sql_validation/corpus_delete_top_level_type.sql

arkindex/sql_validation/corpus_rights_filter.sql

arkindex/sql_validation/corpus_rights_filter_public.sql

arkindex/sql_validation/element_links.sql

arkindex/sql_validation/element_links_not_found.sql

arkindex/sql_validation/list_elements.sql

arkindex/sql_validation/process_elements_filter_type.sql

arkindex/sql_validation/process_elements_top_level.sql

arkindex/sql_validation/process_elements_with_image.sql