Delete task artifacts before deleting tasks in the corpus deletion task

1. Create a corpus
2. Create an element
3. Run a Workers process on the corpus
4. Wait until the initialisation task finishes and creates an artifact
5. Nuke the corpus
6. Watch the deletion job fail when deleting tasks
7. Open the RQ admin

8. django.db.utils.IntegrityError: update or delete on table "ponos_task" violates foreign key constraint "ponos_artifact_task_id_42ccaab3_fk_ponos_task_id" on table "ponos_artifact"
   DETAIL:  Key (id)=(aafda2f4-2e97-4fd7-9c72-e536bdc577e2) is still referenced from table "ponos_artifact".

9. Cry