eCommerce enhancements

However, there seems to be a slightly bigger issue.  It seems to me that you want to enforce the unique constraint that only for all coupons with the same code, only one can be active at once.  I'd ordinarily want to enforce that via a database uniqueness index, except a `unique_together` on `(code, is_active)` then limits you to only 1 active and 1 inactive instances per code.  So I guess checking is fine.

But, then the issue is, shouldn't you also enforce this constraint in `add_coupon`, not just in `update_coupon`?