[fix bug 1007077] Implement email backend for remo dev/stage.

* Use DatabaseEmailBackend to store emails in the database.
* Override this functionality for emails originating from SERVER_EMAIL
  and send them through SMTP to notify ADMINS.
Merge pull request #776 from johngian/1007077-fix-admin-emails

[fix bug 1007077] Implement email backend for remo dev/stage.