Upgrade to Django 3.2

[cash]

1. Need to wait until the autocomplete filter works with django 3.2 - see #160
2. The default primary key type should be set: https://dev.to/weplayinternet/upgrading-to-django-3-2-and-fixing-defaultautofield-warnings-518n
3. default app config doesn't need to be set anymore: https://docs.djangoproject.com/en/3.2/releases/3.2/#automatic-appconfig-discovery

A quick way to find issues is to install with the new django version and then run python -Wa manage.py test