Move command classes to domain namespaces

They will probably be refactored away at a later stage (when we get
rid of the command bus). Until then, this lets us remove the
Flarum\Core namespace and actually feels quite clean.

src/Notification/Command/ReadNotificationHandler.php

@@ -0,0 +1,45 @@
+<?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\Notification\Command;
+
+use Flarum\Notification\Notification;
+use Flarum\User\AssertPermissionTrait;
+
+class ReadNotificationHandler
+{
+    use AssertPermissionTrait;
+
+    /**
+     * @param ReadNotification $command
+     * @return \Flarum\Notification\Notification
+     * @throws \Flarum\User\Exception\PermissionDeniedException
+     */
+    public function handle(ReadNotification $command)
+    {
+        $actor = $command->actor;
+
+        $this->assertRegistered($actor);
+
+        $notification = Notification::where('user_id', $actor->id)->findOrFail($command->notificationId);
+
+        Notification::where([
+            'user_id' => $actor->id,
+            'type' => $notification->type,
+            'subject_id' => $notification->subject_id
+        ])
+            ->update(['is_read' => true]);
+
+        $notification->is_read = true;
+
+        return $notification;
+    }
+}