Merge release 1.6.6

VERSION

...ndex/documents/migrations/0013_corpus_maximum_task_ttl.py

arkindex/documents/models.py

arkindex/documents/serializers/elements.py

arkindex/documents/tests/test_corpus.py

arkindex/ponos/admin.py

arkindex/ponos/api.py

arkindex/ponos/migrations/0012_task_started_finished.py

arkindex/ponos/models.py

arkindex/ponos/serializers.py

arkindex/ponos/tasks.py

arkindex/ponos/tests/rq/test_run_docker_task.py

arkindex/ponos/tests/tasks/__init__.py

arkindex/ponos/tests/tasks/test_partial_update.py

arkindex/ponos/tests/tasks/test_restart.py

arkindex/ponos/tests/tasks/test_retrieve.py

arkindex/ponos/tests/tasks/test_update.py

arkindex/process/api.py

arkindex/process/builder.py

...ndex/process/management/commands/update_system_workers.py