Add configuration for sendmail

If `GITLAB_SENDMAIL` is set to `true` then postfix will be started
and Gitlab will be configured to use sendmail. If both `GITLAB_SENDMAIL`
and `GITLAB_SMTP` are `true` then the sendmail feature is disabled. The
configuration for postfix is pretty basic; it is setup as a relay.

Postfix forks into the background and as a result cannot be started
under supervisord. See the following message for more background
information:

https://groups.google.com/d/msg/mailing.postfix.users/Hwa5673nDUg/Ie9hfmG0OJQJ