Laravel components v8 (#2576)

- update actions ci
- include json for 4 spaces tab
- provide output int for process code exit
- adhere to parent type hint of builder
- mailer instance now needs a name, multiple can be instantiated
- getOriginal now uses mutators in the model
- Temporarily loosen MailableInterface requirements. This avoids an immediate BC break for classes in extensions that implement this interface.
- Temporarily provide (and autoload) old symfony translator interface
- make queue exception handler compatible with the contract of L8
- Update phpunit schema for newer version
- Update phpunit assert calls for newer version

src/Notification/NotificationMailer.php

@@ -12,7 +12,7 @@ namespace Flarum\Notification;
 use Flarum\User\User;
 use Illuminate\Contracts\Mail\Mailer;
 use Illuminate\Mail\Message;
-use Symfony\Component\Translation\TranslatorInterface;
+use Symfony\Contracts\Translation\TranslatorInterface;
 
 class NotificationMailer
 {