Test to see if a better coupon exists already

If we don't find the valid promo in all our places it could be
successfully applied, but it *is* still on the order, it was accepted by
the order but a better one was used instead
Feels like we should downcase on the left too? e.g.

```ruby
x.code.to_s.downcase == order.coupon_code.downcase
```
This also makes me wonder why we don't have a scope in Promotion for coupons only. Will add one