Changing COUPONS_COUPON_TYPES generates additional migrations

[dmarcelino]

Hi,

If the setting COUPONS_COUPON_TYPES is set it will generate additional django-coupons migrations, because it's being used as an input to modelfield type in models.py#L73:

type = models.CharField(_("Type"), max_length=20, choices=COUPON_TYPES)

This is not ideal as it can break a user app if django-coupons add migrations in the future. I suggest choices are only applied to form and not the coupon model.