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/Discussion/Command/DeleteDiscussionHandler.php

@@ -0,0 +1,64 @@
+<?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\Discussion\Command;
+
+use Flarum\Discussion\DiscussionRepository;
+use Flarum\Discussion\Event\Deleting;
+use Flarum\Foundation\DispatchEventsTrait;
+use Flarum\User\AssertPermissionTrait;
+use Flarum\User\Exception\PermissionDeniedException;
+use Illuminate\Contracts\Events\Dispatcher;
+
+class DeleteDiscussionHandler
+{
+    use DispatchEventsTrait;
+    use AssertPermissionTrait;
+
+    /**
+     * @var \Flarum\Discussion\DiscussionRepository
+     */
+    protected $discussions;
+
+    /**
+     * @param Dispatcher $events
+     * @param DiscussionRepository $discussions
+     */
+    public function __construct(Dispatcher $events, DiscussionRepository $discussions)
+    {
+        $this->events = $events;
+        $this->discussions = $discussions;
+    }
+
+    /**
+     * @param DeleteDiscussion $command
+     * @return \Flarum\Discussion\Discussion
+     * @throws PermissionDeniedException
+     */
+    public function handle(DeleteDiscussion $command)
+    {
+        $actor = $command->actor;
+
+        $discussion = $this->discussions->findOrFail($command->discussionId, $actor);
+
+        $this->assertCan($actor, 'delete', $discussion);
+
+        $this->events->fire(
+            new Deleting($discussion, $actor, $command->data)
+        );
+
+        $discussion->delete();
+
+        $this->dispatchEventsFor($discussion, $actor);
+
+        return $discussion;
+    }
+}