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/Notification/Blueprint/AbstractBlueprint.php

@@ -0,0 +1,23 @@
+<?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\Notification\Blueprint;
+
+abstract class AbstractBlueprint implements BlueprintInterface
+{
+    public function getAttributes(): array
+    {
+        return [
+            'type' => static::getType(),
+            'from_user_id' => ($fromUser = $this->getFromUser()) ? $fromUser->id : null,
+            'subject_id' => ($subject = $this->getSubject()) ? $subject->getKey() : null,
+            'data' => ($data = $this->getData()) ? json_encode($data) : null
+        ];
+    }
+}

src/Notification/Blueprint/BlueprintInterface.php

@@ -9,6 +9,9 @@
 
 namespace Flarum\Notification\Blueprint;
 
+use Flarum\Database\AbstractModel;
+use Flarum\User\User;
+
 /**
  * A notification BlueprintInterface, when instantiated, represents a notification about
  * something. The blueprint is used by the NotificationSyncer to commit the
@@ -19,14 +22,14 @@ interface BlueprintInterface
     /**
      * Get the user that sent the notification.
      *
-     * @return \Flarum\User\User|null
+     * @return User|null
      */
     public function getFromUser();
 
     /**
      * Get the model that is the subject of this activity.
      *
-     * @return \Flarum\Database\AbstractModel|null
+     * @return AbstractModel|null
      */
     public function getSubject();