Disallow editing if discussions are locked

Fixes flarum/core#1156.

extensions/lock/bootstrap.php

@@ -21,4 +21,5 @@ return function (Dispatcher $events) {
     $events->subscribe(Listener\SaveLockedToDatabase::class);
 
     $events->subscribe(Access\DiscussionPolicy::class);
+    $events->subscribe(Access\PostPolicy::class);
 };

extensions/lock/src/Access/PostPolicy.php

@@ -0,0 +1,37 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Lock\Access;
+
+use Flarum\Core\Access\AbstractPolicy;
+use Flarum\Core\User;
+use Flarum\Core\Post;
+
+class PostPolicy extends AbstractPolicy
+{
+    /**
+     * {@inheritdoc}
+     */
+    protected $model = Post::class;
+
+    /**
+     * @param User $actor
+     * @param Post $post
+     * @return bool
+     */
+    public function edit(User $actor, Post $post)
+    {
+        $discussion = $post->discussion;
+        if ($discussion->is_locked && $actor->cannot('lock', $discussion)) {
+            return false;
+        }
+    }
+}