Extract current user attributes into a separate serializer

This prevents the unread notifications count query being run for every
post by the currently authenticated user

src/Api/Serializers/CurrentUserSerializer.php

@@ -0,0 +1,21 @@
+<?php namespace Flarum\Api\Serializers;
+
+class CurrentUserSerializer extends UserSerializer
+{
+    protected function attributes($user)
+    {
+        $attributes = parent::attributes($user);
+
+        $actingUser = $this->actor->getUser();
+
+        if ($user->id === $actingUser->id) {
+            $attributes += [
+                'readTime' => $user->read_time ? $user->read_time->toRFC3339String() : null,
+                'unreadNotificationsCount' => $user->getUnreadNotificationsCount(),
+                'preferences' => $user->preferences
+            ];
+        }
+
+        return $this->extendAttributes($user, $attributes);
+    }
+}