Add corpus thumbnail

arkindex/documents/api/elements.py

arkindex/documents/migrations/0033_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_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/element_trash_children.sql

arkindex/sql_validation/element_trash_deep.sql

arkindex/sql_validation/element_trash_no_children.sql

arkindex/sql_validation/list_elements.sql