Added OfferRedemption model

refs https://github.com/TryGhost/Team/issues/1132

A model is needed for reading and writing Offer Redemptions to the
database.

core/server/models/index.js

@@ -30,6 +30,7 @@ const models = [
     'mobiledoc-revision',
     'member',
     'offer',
+    'offer-redemption',
     'product',
     'benefit',
     'stripe-product',

core/server/models/offer-redemption.js

@@ -0,0 +1,10 @@
+const ghostBookshelf = require('./base');
+
+const OfferRedemption = ghostBookshelf.Model.extend({
+    tableName: 'offer_redemptions'
+});
+
+module.exports = {
+    OfferRedemption: ghostBookshelf.model('OfferRedemption', OfferRedemption)
+};
+