Fixed Stripe Checkout using Offers

refs https://github.com/TryGhost/Members/commit/5172e40646

When we updated to use the OffersAPI instead of OfferRepository this was
missed, and we were passing blank coupon to Stripe Checkout. This should
eventually be replaced with a call like `getCoupon(offerId)` from a
payments service.

ghost/members-api/lib/controllers/router.js

@@ -152,7 +152,7 @@ module.exports = class RouterController {
                 }
 
                 coupon = {
-                    id: offer.stripeCouponId
+                    id: offer.stripe_coupon_id
                 };
             } catch (err) {
                 res.writeHead(500);

ghost/offers/lib/application/OfferMapper.js

@@ -24,6 +24,8 @@
  *
  * @prop {'active'|'archived'} status
  *
+ * @prop {string} stripe_coupon_id
+ *
  * @prop {object} tier
  * @prop {string} tier.id
  * @prop {string} tier.name
@@ -49,6 +51,7 @@ class OfferMapper {
             currency_restriction: offer.type.value === 'fixed',
             currency: offer.type.value === 'fixed' ? offer.currency.value : null,
             status: offer.status.value,
+            stripe_coupon_id: offer.stripeCouponId,
             tier: {
                 id: offer.tier.id,
                 name: offer.tier.name