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/Group/Command/CreateGroupHandler.php

@@ -0,0 +1,72 @@
+<?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\Group\Command;
+
+use Flarum\Foundation\DispatchEventsTrait;
+use Flarum\Group\Event\Saving;
+use Flarum\Group\Group;
+use Flarum\Group\GroupValidator;
+use Flarum\User\AssertPermissionTrait;
+use Illuminate\Contracts\Events\Dispatcher;
+
+class CreateGroupHandler
+{
+    use DispatchEventsTrait;
+    use AssertPermissionTrait;
+
+    /**
+     * @var \Flarum\Group\GroupValidator
+     */
+    protected $validator;
+
+    /**
+     * @param Dispatcher $events
+     * @param \Flarum\Group\GroupValidator $validator
+     */
+    public function __construct(Dispatcher $events, GroupValidator $validator)
+    {
+        $this->events = $events;
+        $this->validator = $validator;
+    }
+
+    /**
+     * @param CreateGroup $command
+     * @return \Flarum\Group\Group
+     * @throws \Flarum\User\Exception\PermissionDeniedException
+     */
+    public function handle(CreateGroup $command)
+    {
+        $actor = $command->actor;
+        $data = $command->data;
+
+        $this->assertCan($actor, 'createGroup');
+
+        $group = Group::build(
+            array_get($data, 'attributes.nameSingular'),
+            array_get($data, 'attributes.namePlural'),
+            array_get($data, 'attributes.color'),
+            array_get($data, 'attributes.icon')
+        );
+
+        $this->events->fire(
+            new Saving($group, $actor, $data)
+        );
+
+        $this->validator->assertValid($group->getAttributes());
+
+        $group->save();
+
+        $this->dispatchEventsFor($group, $actor);
+
+        return $group;
+    }
+}