Added stripeCouponId to Offer

refs https://github.com/TryGhost/Team/issues/1090

When creating a Stripe Checkout Session for an Offer - we need access to
the underlying Stripe Coupon. Exposing it here allows consumers of the
OfferRepository access.

ghost/offers/lib/Offer.js

@@ -12,6 +12,7 @@ const ObjectID = require('bson-objectid').default;
  * @prop {'percent'|'amount'} type
  * @prop {number} amount
  * @prop {string} currency
+ * @prop {string} [stripe_coupon_id]
  * @prop {OfferTier} tier
  */
 
@@ -167,6 +168,10 @@ class Offer {
         return !!this.options.isNew;
     }
 
+    get stripeCouponId() {
+        return this.props.stripe_coupon_id;
+    }
+
     /**
      * @private
      * @param {OfferProps} props
@@ -291,6 +296,18 @@ class Offer {
         const cadence = data.cadence;
         const currency = data.currency;
 
+        if (isNew && data.stripe_coupon_id) {
+            throw new errors.InvalidOfferCoupon({
+                message: 'Cannot supply a stripe_coupon_id for new Offers.'
+            });
+        }
+        if (!isNew && !data.stripe_coupon_id) {
+            throw new errors.InvalidOfferCoupon({
+                message: 'Offers must have a stripe_coupon_id.'
+            });
+        }
+        const couponId = data.stripe_coupon_id;
+
         const tier = OfferTier.create(data.tier);
 
         return new Offer({
@@ -303,7 +320,8 @@ class Offer {
             amount,
             cadence,
             currency,
-            tier
+            tier,
+            stripe_coupon_id: couponId
         }, {isNew});
     }
 }

ghost/offers/lib/OfferRepository.js

@@ -22,6 +22,7 @@ function toDomain(json) {
         amount: json.discount_amount,
         cadence: json.interval,
         currency: json.currency,
+        stripe_coupon_id: json.stripe_coupon_id,
         tier: {
             id: json.product.id,
             name: json.product.name

ghost/offers/lib/errors.js

@@ -23,6 +23,7 @@ class InvalidOfferAmount extends InvalidPropError {}
 class InvalidOfferCurrency extends InvalidPropError {}
 class InvalidOfferTierName extends InvalidPropError {}
 class InvalidOfferCadence extends InvalidPropError {}
+class InvalidOfferCoupon extends InvalidPropError {}
 
 module.exports = {
     InvalidOfferNameError,
@@ -33,5 +34,6 @@ module.exports = {
     InvalidOfferAmount,
     InvalidOfferCurrency,
     InvalidOfferCadence,
-    InvalidOfferTierName
+    InvalidOfferTierName,
+    InvalidOfferCoupon
 };