With upcoming changes to the dovecot home and maildirectories we need to
introduce a way to nudge users to inform themselves about manual
migration steps they might need to carry out.
The idea here is to allow us to safely make breaking changes and notify
the user of required migration steps at eval time, so they can make the
necessary changes in time.
Allow the user to specify the name of the ACME configuration that the
mailserver should use. This allows users that request certificates that
aren't the FQDN of the mailserver, for example a wildcard certificate.
