Disabled Collections sync when flag disabled

We've been seeing an increased number of database queries, which are potentially
caused by the collections syncing logic. Whilst we haven't narrowed the cause
down precisely we want to disable this logic unless the flag is enabled.

ghost/core/core/server/services/collections/service.js

@@ -2,6 +2,7 @@ const {
     CollectionsService
 } = require('@tryghost/collections');
 const BookshelfCollectionsRepository = require('./BookshelfCollectionsRepository');
+const labs = require('../../../shared/labs');
 
 let inited = false;
 class CollectionsServiceWrapper {
@@ -31,6 +32,9 @@ class CollectionsServiceWrapper {
     }
 
     async init() {
+        if (!labs.isSet('collections')) {
+            return;
+        }
         if (inited) {
             return;
         }