API: Add event for determining user groups for permissions

src/Events/GetUserGroups.php

@@ -0,0 +1,29 @@
+<?php namespace Flarum\Events;
+
+use Flarum\Core\Users\User;
+
+/**
+ * The `GetUserGroups` event
+ */
+class GetUserGroups
+{
+    /**
+     * @var User
+     */
+    public $user;
+
+    /**
+     * @var array
+     */
+    public $groupIds;
+
+    /**
+     * @param User $user
+     * @param array $groupIds
+     */
+    public function __construct(User $user, array &$groupIds)
+    {
+        $this->user = $user;
+        $this->groupIds = &$groupIds;
+    }
+}