Notifications: Reorgranised classes into domain specific folders

Closes #4500

app/Access/Notifications/UserInviteNotification.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace BookStack\Access\Notifications;
+
+use BookStack\App\MailNotification;
+use BookStack\Users\Models\User;
+use Illuminate\Notifications\Messages\MailMessage;
+
+class UserInviteNotification extends MailNotification
+{
+    public function __construct(
+        public string $token
+    ) {
+    }
+
+    public function toMail(User $notifiable): MailMessage
+    {
+        $appName = ['appName' => setting('app-name')];
+        $language = $notifiable->getLanguage();
+
+        return $this->newMailMessage($language)
+                ->subject(trans('auth.user_invite_email_subject', $appName, $language))
+                ->greeting(trans('auth.user_invite_email_greeting', $appName, $language))
+                ->line(trans('auth.user_invite_email_text', [], $language))
+                ->action(trans('auth.user_invite_email_action', [], $language), url('/register/invite/' . $this->token));
+    }
+}