Delete training links upon element deletion

arkindex/documents/deletion.py

arkindex/documents/managers.py

arkindex/sql_validation/element_dot_delete.sql

arkindex/sql_validation/element_trash_children.sql

arkindex/sql_validation/element_trash_deep.sql

arkindex/sql_validation/element_trash_ml_class.sql

arkindex/sql_validation/element_trash_no_children.sql

...dex/sql_validation/worker_results_delete_all_versions.sql

arkindex/sql_validation/worker_results_delete_in_corpus.sql

...dex/sql_validation/worker_results_delete_under_parent.sql

...alidation/worker_results_delete_under_parent_included.sql