Switch to NixOS ACME module for certificate management

Drop most of the existing certificate handling, because we're effectively
duplicating functionality that NixOS offers for free with better
design, testing and maintainance than what we could provide downstream.

The remaining two options are to reference an
existing `security.acme.certs` configuration through
`mailserver.x509.useACMEHost` or to provide existing key material via
`mailserver.x509.certificateFile` and `mailserver.x509.privateKeyFile`.

Support for automatic creation of self-signed certificates has been
removed, because it is undesirable in public mail setups.

The updated setup guide now displays the recommended configuration that
relies on the NixOS ACME module, but requires further customization to
select a suitable challenge.

Co-Authored-By: Emily <git@emilylange.de>

mail-server/common.nix

@@ -24,28 +24,20 @@
 let
   cfg = config.mailserver;
 in
-{
-  # cert :: PATH
-  certificatePath =
-    if cfg.certificateScheme == "manual" then
-      cfg.certificateFile
-    else if cfg.certificateScheme == "selfsigned" then
-      "${cfg.certificateDirectory}/cert-${cfg.fqdn}.pem"
-    else if cfg.certificateScheme == "acme" || cfg.certificateScheme == "acme-nginx" then
-      "${config.security.acme.certs.${cfg.acmeCertificateName}.directory}/fullchain.pem"
-    else
-      throw "unknown certificate scheme";
+rec {
+  withACME = cfg.x509.useACMEHost != null;
 
-  # key :: PATH
-  keyPath =
-    if cfg.certificateScheme == "manual" then
-      cfg.keyFile
-    else if cfg.certificateScheme == "selfsigned" then
-      "${cfg.certificateDirectory}/key-${cfg.fqdn}.pem"
-    else if cfg.certificateScheme == "acme" || cfg.certificateScheme == "acme-nginx" then
-      "${config.security.acme.certs.${cfg.acmeCertificateName}.directory}/key.pem"
+  x509CertificateFile =
+    if withACME then
+      "${config.security.acme.certs.${cfg.x509.useACMEHost}.directory}/fullchain.pem"
     else
-      throw "unknown certificate scheme";
+      cfg.x509.certificateFile;
+
+  x509PrivateKeyFile =
+    if withACME then
+      "${config.security.acme.certs.${cfg.x509.useACMEHost}.directory}/key.pem"
+    else
+      cfg.x509.privateKeyFile;
 
   passwordFiles =
     let