Add exclusion constraints on ElementPath ordering

arkindex/documents/api/elements.py

arkindex/documents/api/ml.py

arkindex/documents/fixtures/data.json

...ndex/documents/migrations/0065_elementpath_constraints.py

arkindex/documents/models.py

...dex/documents/tests/commands/test_move_lines_to_parent.py

arkindex/documents/tests/tasks/test_move_element.py

arkindex/documents/tests/test_bulk_element_transcriptions.py

arkindex/documents/tests/test_bulk_elements.py

arkindex/documents/tests/test_children_elements.py

arkindex/documents/tests/test_create_elements.py

arkindex/documents/tests/test_edit_elementpath.py

arkindex/documents/tests/test_element_paths_api.py

arkindex/documents/tests/test_neighbors.py

arkindex/sql_validation/add_first_parent.sql

arkindex/sql_validation/add_second_parent.sql

arkindex/sql_validation/element_move_with_children.sql

arkindex/sql_validation/element_move_without_child.sql

arkindex/sql_validation/remove_child_last_parent.sql