[FIX] fix bug with standard emails sending

fast-track of a fix from @invitu that we lost by error. Ref: https://github.com/Antiun/social/pull/2

mass_mailing_statistic_extra/models/mail_mail.py

@@ -30,4 +30,4 @@ class MailMail(models.Model):
                     'email_to': ';'.join(email_list),
                     'subject': mail.subject,
                 })
-        return mail
+        return mail_id