Notifications into the queue

Forces notifications into a dedicated SendNotificationsJob and passed
to the queue.

- One static method re-used in the job ::getAttributes, is that okay or
  use a trait?
- Do we want to use this solution and refactor into a better Hub after
  stable, postpone this implementation or use it in b11?

src/Queue/AbstractJob.php

@@ -0,0 +1,22 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Queue;
+
+use Illuminate\Bus\Queueable;
+use Illuminate\Contracts\Queue\ShouldQueue;
+use Illuminate\Queue\InteractsWithQueue;
+use Illuminate\Queue\SerializesModels;
+
+class AbstractJob implements ShouldQueue
+{
+    use InteractsWithQueue;
+    use Queueable;
+    use SerializesModels;
+}