Notifications: Aligned how user language is used

- This ensures content notifications are not translated to receiver
  language.
- This adds actual plaintext support for content notifications (Was
  previously just HTML as text view).
- Shares same base class across all mail notifications.
- Also cleaned up existing notification classes.

Future cleanup requested via #4501

app/Users/Models/User.php

@@ -11,6 +11,7 @@ use BookStack\App\Model;
 use BookStack\App\Sluggable;
 use BookStack\Entities\Tools\SlugGenerator;
 use BookStack\Notifications\ResetPassword;
+use BookStack\Translation\LanguageManager;
 use BookStack\Uploads\Image;
 use Carbon\Carbon;
 use Exception;
@@ -338,6 +339,14 @@ class User extends Model implements AuthenticatableContract, CanResetPasswordCon
         return '';
     }
 
+    /**
+     * Get the system language for this user.
+     */
+    public function getLanguage(): string
+    {
+        return app()->make(LanguageManager::class)->getLanguageForUser($this);
+    }
+
     /**
      * Send the password reset notification.
      *