mail: push registered mail number to formdata submission context (#10945)

welco/sources/mail/models.py

@@ -109,6 +109,7 @@ class Mail(models.Model):
         return {
             'channel': 'mail',
             'post_date': self.post_date and self.post_date.strftime('%Y-%m-%d'),
+            'registered_mail_number': self.registered_mail_number,
         }