Deferred heavy permittedAttributes call unless needed

- this code is on the hotpath for the URL service and has shown to be
  slow for sites with a lot of posts
- this is due to the overhead of the lodash functions we use here
- we can take advantage of how JS executes if-statements and move the
  variable into the if-statement, which lazy evaluates it (for the URL
  service, this branch is not hit, so it's a big win)
- this cuts about 2% from CPU time

ghost/core/core/server/models/post.js

@@ -1290,11 +1290,13 @@ Post = ghostBookshelf.Model.extend({
             options.withRelated = _.union(['authors', 'tags', 'post_revisions', 'post_revisions.author'], options.withRelated || []);
         }
 
-        const META_ATTRIBUTES = _.without(ghostBookshelf.model('PostsMeta').prototype.permittedAttributes(), 'id', 'post_id');
-
         // NOTE: only include post_meta relation when requested in 'columns' or by default
         //       optimization is needed to be able to perform .findAll on large SQLite datasets
-        if (!options.columns || (options.columns && _.intersection(META_ATTRIBUTES, options.columns).length)) {
+        if (!options.columns
+        || (
+            options.columns
+            && _.intersection(_.without(ghostBookshelf.model('PostsMeta').prototype.permittedAttributes(), 'id', 'post_id'), options.columns).length)
+        ) {
             options.withRelated = _.union(['posts_meta'], options.withRelated || []);
         }