Added currentCountQuery for newsletters

refs TryGhost/Product#1583

Author: aileen

packages/limit-service/lib/config.js

@@ -9,7 +9,16 @@ module.exports = {
             return result.count;
         }
     },
-    newsletters: {},
+    newsletters: {
+        currentCountQuery: async (db) => {
+            let result = await db.knex('newsletters')
+                .count('id', {as: 'count'})
+                .where('status', '=', 'active')
+                .first();
+
+            return result.count;
+        }
+    },
     emails: {
         currentCountQuery: async (db, startDate) => {
             let result = await db.knex('emails')