Set default auto field

[sebastianhaas]

In order to avoid a database migration, we set default_auto_field to 'django.db.models.BigAutoField' for the VerifyEmail app. This is required, since Django defaults to django.db.models.AutoField since Django 3.2 (https://docs.djangoproject.com/en/3.2/ref/settings/#std-setting-DEFAULT_AUTO_FIELD).

By setting default_auto_field to the big auto field, the VerifyEmail app sticks with the existing big auto field in the database.