change Job.input_json and output_json to use Django 3.1 models.JSONField

[matthewcornell]

We need to bring these two fields up to date with Django 3.1's feature. I did not do this as part of the big Zoltar 4 upgrade because we were focusing on migrating forecast data at that time to keep things as simple as possible.

• migrate current Jobs - see migrations.RunPython(...) in 0010_forecast_issue_date.py
• ensure forecast_app.serializers.JobSerializer() works - see note per https://github.com/dmkoch/django-jsonfield/issues/188
• pipenv uninstall jsonfield