Migrate all primary key fields from AutoField to BigAutoField

[StephenChan]

So we don't have to worry about stuff like issue #447 in the future, and to make all the tables consistent.

This would be easier to do in Django 3.2 or later: https://docs.djangoproject.com/en/dev/releases/3.2/#customizing-type-of-auto-created-primary-keys