Added monthly & yearly price relation to Product

refs https://github.com/TryGhost/Team/issues/710

This allows us to fetch the default monthly and yearly price models for
a product model, which is important since we no longer want to expose
the entire list of prices, but just the designated monthly & yearly prices.

core/server/models/product.js

@@ -25,6 +25,14 @@ const Product = ghostBookshelf.Model.extend({
         }
     },
 
+    monthlyPrice() {
+        return this.belongsTo('StripePrice', 'monthly_price_id', 'id');
+    },
+
+    yearlyPrice() {
+        return this.belongsTo('StripePrice', 'yearly_price_id', 'id');
+    },
+
     stripeProducts() {
         return this.hasMany('StripeProduct', 'product_id', 'id');
     },