Clean ponos DB fields

arkindex/documents/fixtures/data.json

arkindex/ponos/__init__.py

arkindex/ponos/fields.py

arkindex/ponos/forms.py

arkindex/ponos/helpers.py

arkindex/ponos/migrations/0009_tags.py

arkindex/ponos/migrations/0012_advanced_tags.py

arkindex/ponos/migrations/0014_modify_task_model.py

arkindex/ponos/migrations/0031_emptyable_jsonfield.py

arkindex/ponos/migrations/0035_alter_task_tags.py

.../ponos/migrations/0036_hstore_task_env_and_extra_files.py

arkindex/ponos/models.py

arkindex/ponos/tests/test_util.py