Merge #1890

1890: fix Email class in model.py r=mergify[bot] a=ghostwheel42

## What type of PR?

bug-fix

## What does this PR do?

fixes class Email - keep email, localpart and domain in sync.

### Related issue(s)

closes #1878


Co-authored-by: Alexander Graf <ghostwheel42@users.noreply.github.com>

core/admin/mailu/models.py

@@ -371,8 +371,8 @@ class Email(object):
     @classmethod
     def __declare_last__(cls):
         # gets called after mappings are completed
-        sqlalchemy.event.listen(User.localpart, 'set', cls._update_localpart, propagate=True)
+        sqlalchemy.event.listen(cls.localpart, 'set', cls._update_localpart, propagate=True)
-        sqlalchemy.event.listen(User.domain_name, 'set', cls._update_domain_name, propagate=True)
+        sqlalchemy.event.listen(cls.domain_name, 'set', cls._update_domain_name, propagate=True)
 
     def sendmail(self, subject, body):
         """ send an email to the address """