Replace rspamd with OpenDKIM

OpenDKIM configuration
has two Lua scripts defining strict DKIM policy.

screen.lua filters out signatures that do not correspond
to the From: domain so they are not even checked.
final.lua rejects mail if it is not outgoing
and has no valid DKIM signatures.

OpenDKIM is configured as a milter on port 25 smtpd
to check DKIM signatures
and on mail reinjecting smtpd
to sign outgoing messages with DKIM signatures.

cmdeploy/src/cmdeploy/postfix/main.cf.j2

@@ -45,6 +45,3 @@ inet_protocols = all
 
 virtual_transport = lmtp:unix:private/dovecot-lmtp
 virtual_mailbox_domains = {{ config.mail_domain }}
-
-smtpd_milters = inet:127.0.0.1:11332
-non_smtpd_milters = $smtpd_milters