Allow to enter GiftCard code in the same input as coupon

[Roshyo]

It would be useful to allow users to enter the Gift card code in the same input as regular coupon. It should also be a possibility, not the unique behavior. So we could add in admin panel a new configuration for GiftCard that would be like "Have 2 separate inputs in cart ?"

If yes : keep behavior like now
If no : Hook on "coupon application" and check whether it's a coupon (default behavior) or a Gift Card code.

We would also need to add a validator to GiftCardCode so they are never the same as coupon

[loevgaard]

For the first version of this feature I don't think we should have the toggle in the backend, but just as a configuration option.