Replace usage of django-postgres-set-field with JSONB

[francisli]

Since django-postgres-set-field is deprecated

[francisli]

Related to #108

[francisli]

Info on Django Postgres Set Field: https://github.com/CodeYellowBV/django-postgres-setfield

It's used to store a "set" in the db. I think we could replace that with a JSONB array so that we could remove this now-deprecated library?

[arilamstein]

Quick update: This article (written in 2020) says to use from django.contrib.postgres.fields import JSONField: https://pganalyze.com/blog/postgres-jsonb-django-python

However, the code for the JSONField class says to use django.db.models.JSONField instead:

class JSONField(BuiltinJSONField):
    system_check_removed_details = {
        "msg": (
            "django.contrib.postgres.fields.JSONField is removed except for "
            "support in historical migrations."
        ),
        "hint": "Use django.db.models.JSONField instead.",
        "id": "fields.E904",
    }