Group storage and vmail user options at mailserver.storage

Create a nicer option structure that deals with the mail storage and its
owner, uid, group and gid. Also includes the directory layout as a
property of how mails are stored..

mail-server/assertions.nix

@@ -98,22 +98,20 @@ in
         ) config.mailserver.dkim.domains
       )
     )
-    ++
-      lib.optionals (config.mailserver.ldap.enable && config.mailserver.mailDirectory != "/var/vmail")
-        [
-          {
-            assertion = config.mailserver.stateVersion != null -> config.mailserver.stateVersion >= 2;
-            message = ''
-              Issue: The dovecot homedir for LDAP users was previously not respecting `mailserver.mailDirectory`.
-              Remediation:
-                - Stop the `dovecot.service`
-                - Move `/var/vmail/ldap` below your `mailserver.mailDirectory`
-                - Increase the `stateVersion` to 2.
+    ++ lib.optionals (config.mailserver.ldap.enable && config.mailserver.storage.path != "/var/vmail") [
+      {
+        assertion = config.mailserver.stateVersion != null -> config.mailserver.stateVersion >= 2;
+        message = ''
+          Issue: The dovecot homedir for LDAP users was previously not respecting `mailserver.storage.path`.
+          Remediation:
+            - Stop the `dovecot.service`
+            - Move `/var/vmail/ldap` below your `mailserver.storage.path`
+            - Increase the `stateVersion` to 2.
 
-              Check https://nixos-mailserver.readthedocs.io/en/latest/migrations.html#dovecot-ldap-home-directory-migration for more information.
-            '';
-          }
-        ]
+          Check https://nixos-mailserver.readthedocs.io/en/latest/migrations.html#dovecot-ldap-home-directory-migration for more information.
+        '';
+      }
+    ]
     ++ [
       {
         assertion = config.mailserver.stateVersion != null -> config.mailserver.stateVersion >= 3;