Update to Django 3

[helrond]

Is your feature request related to a problem? Please describe.

Once all apps which use this library are updated to Django 3, we'll need to update this library to use Django 3 as well.

Describe the solution you'd like

A couple of specific issues:

• django.contrib.postgres.fields.JSONField is deprecated, use django.db.models.JSONField instead.
• Explicitly set the primary key type: https://docs.djangoproject.com/en/3.2/releases/3.2/#customizing-type-of-auto-created-primary-keys