Added 'getDefaultProduct' convenience method to product repo

refs TryGhost/Product#1869

- There are multiple places in the codebase fetching "default product". The code is slightly divergent in each one of them and has been a source of bugs (like the one referenced). Having the logic captured in one place will allow reducing the code duplication, making code less bug prone, and making testing the modules dependent on the "setDefaultProduct" method easier

ghost/members-api/lib/repositories/product.js

@@ -128,6 +128,21 @@ class ProductRepository {
         throw new NotFoundError({message: 'Missing id, slug, stripe_product_id or stripe_price_id from data'});
     }
 
+    /**
+     * Fetches the default product
+     * @param {Object} options
+     * @returns {Promise<ProductModel>}
+     */
+    async getDefaultProduct(options = {}) {
+        const defaultProductPage = await this.list({
+            filter: 'type:paid+active:true',
+            limit: 1,
+            ...options
+        });
+
+        return defaultProductPage.data[0];
+    }
+
     /**
      * Creates a product from a name
      *

ghost/members-api/test/unit/lib/repositories/product.test.js

@@ -0,0 +1,28 @@
+const assert = require('assert');
+const sinon = require('sinon');
+const ProductRepository = require('../../../../lib/repositories/product');
+
+describe('MemberRepository', function () {
+    describe('getDefaultProduct', function () {
+        it('calls list method with specific parameters', async function () {
+            const productRepository = new ProductRepository({});
+            const listStub = sinon.stub(productRepository, 'list').resolves({
+                data: [{
+                    id: 'default_product_id'
+                }]
+            });
+
+            const defaultProduct = await productRepository.getDefaultProduct({
+                withRelated: ['stripePrices']
+            });
+
+            assert.ok(listStub.called);
+
+            assert.equal(listStub.args[0][0].filter, 'type:paid+active:true', 'should only take into account paid and active records');
+            assert.equal(listStub.args[0][0].limit, 1, 'should only fetch a single record');
+            assert.deepEqual(listStub.args[0][0].withRelated, ['stripePrices'], 'should extend passed in options');
+
+            assert.equal(defaultProduct.id, 'default_product_id', 'returns a single product object');
+        });
+    });
+});