Notifications: Reorgranised classes into domain specific folders

Closes #4500

app/Settings/TestEmailNotification.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace BookStack\Settings;
+
+use BookStack\App\MailNotification;
+use BookStack\Users\Models\User;
+use Illuminate\Notifications\Messages\MailMessage;
+
+class TestEmailNotification extends MailNotification
+{
+    public function toMail(User $notifiable): MailMessage
+    {
+        return $this->newMailMessage()
+                ->subject(trans('settings.maint_send_test_email_mail_subject'))
+                ->greeting(trans('settings.maint_send_test_email_mail_greeting'))
+                ->line(trans('settings.maint_send_test_email_mail_text'));
+    }
+}