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.
Add a certificate scheme for using an existing ACME certificate without
setting up Nginx.
Also use names instead of magic numbers for certificate schemes.
- Move the "create password hash file from hashed password" behavior to
a separate variable, since having it in the default field of config
would always cause the warning to trigger
- Change type of hashedPassword to `nullOr str`
