Full SQL implementation of Element.add_parent and remove_child

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_edit_elementpath.py

arkindex/documents/tests/test_element_paths_api.py

arkindex/project/fields.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.sql

arkindex/sql_validation/remove_child_last_parent.sql