Added periodic "emails" limit check when (re)sending a newsletter

refs https://github.com/TryGhost/Team/issues/588
refs d72ba77aba

- When limit is in place we don't want to allow sending out a new batch of emails if it would go over limit
- See referenced commit for example configuration

core/server/services/mega/mega.js

@@ -140,6 +140,10 @@ const addEmail = async (postModel, options) => {
         return null;
     }
 
+    if (limitService.isLimited('emails')) {
+        await limitService.errorIfWouldGoOverLimit('emails', {addedCount: membersCount});
+    }
+
     const postId = postModel.get('id');
     const existing = await models.Email.findOne({post_id: postId}, knexOptions);