Eager load discussion relationships

Since extensions may add nested includes, we need to make sure they are eager-loaded to avoid excessive queries. For example, when the tags extension adds "tags" and "tags.state".

src/Api/Controller/ShowDiscussionController.php

@@ -83,6 +83,10 @@ class ShowDiscussionController extends AbstractResourceController
             $this->includePosts($discussion, $request, $postRelationships);
         }
 
+        $discussion->load(array_filter($include, function ($relationship) {
+            return ! starts_with($relationship, 'posts');
+        }));
+
         return $discussion;
     }