Fixed comped tier assignment

closes https://github.com/TryGhost/Team/issues/1869

- When there were "archived" tiers in the system the importer incorrectly fetched them instead of only taking "active" ones into account. The "getDefaultProduct" on product repository does exactly that.
- Additionally, reusing the "getDefaultProduct" makes testing the importer slightly less complex.

ghost/members-importer/lib/importer.js

@@ -109,12 +109,7 @@ module.exports = class MembersCSVImporter {
 
         const membersApi = await this._getMembersApi();
 
-        const defaultProductPage = await membersApi.productRepository.list({
-            filter: 'type:paid',
-            limit: 1
-        });
-
-        const defaultProduct = defaultProductPage.data[0];
+        const defaultProduct = await membersApi.productRepository.getDefaultProduct();
 
         const result = await rows.reduce(async (resultPromise, row) => {
             const resultAccumulator = await resultPromise;

ghost/members-importer/test/importer.test.js

@@ -54,10 +54,8 @@ describe('Importer', function () {
         });
         membersApiStub = {
             productRepository: {
-                list: async () => {
-                    return {
-                        data: [defaultProduct]
-                    };
+                getDefaultProduct: async () => {
+                    return defaultProduct;
                 }
             },
             members: {