Use attribute for User#external? (#35579)

app/models/user.rb

@@ -143,7 +143,9 @@ class User < ApplicationRecord
   delegate :can?, to: :role
 
   attr_reader :invite_code, :date_of_birth
-  attr_writer :external, :bypass_registration_checks, :current_account
+  attr_writer :bypass_registration_checks, :current_account
+
+  attribute :external, :boolean, default: false
 
   def self.those_who_can(*any_of_privileges)
     matching_role_ids = UserRole.that_can(*any_of_privileges).map(&:id)
@@ -507,10 +509,6 @@ class User < ApplicationRecord
     Setting.registrations_mode == 'open'
   end
 
-  def external?
-    !!@external
-  end
-
   def bypass_registration_checks?
     @bypass_registration_checks
   end