Option to disable coupons at checkout server-side#27

Right now, it’s only possible to hide coupons client-side by appending &disable_promo_codes=1 in the checkout URL, which can easily be seen and removed by customers. It would be nice to be able to completely disallow coupons from the user, but not coupons sepcified by the server when creating the checkout session.

This would allow headless implementations to have their own business logic regarding the eligibility of coupons that go beyond PayNow’s current coupon rules.

Note that gift cards could still be applyable (or make them a separate option to disable them server-side).