Added newsletter relation to activity feed

refs https://github.com/TryGhost/Team/issues/1478

Makes sure the newsletter relation is returned in the activity feed for susbcribe events (aka newsletter events).
Tests in https://github.com/TryGhost/Ghost/pull/14585

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

@@ -30,7 +30,7 @@ module.exports = class EventRepository {
     async getNewsletterSubscriptionEvents(options = {}, filters = {}) {
         options = {
             ...options,
-            withRelated: ['member'],
+            withRelated: ['member', 'newsletter'],
             filter: []
         };
         if (filters['data.created_at']) {