roundcube & mailserver: fix oauth: mailserver is an OAuth secret donor

Both of them use the same client ID and client secret, but Roundcube
depends on mailserver generally, so mailserver is the one to share OAuth
client id and secret.

sp-modules/simple-nixos-mailserver/common.nix

@@ -3,8 +3,9 @@ rec {
   auth-passthru = config.passthru.selfprivacy.auth;
   domain = config.selfprivacy.domain;
   is-auth-enabled = config.selfprivacy.modules.auth.enable or false;
+  group = "dovecot2";
 
-  appendLdapBindPwd =
+  appendSetting =
     { name, file, prefix, suffix ? "", passwordFile, destination }:
     pkgs.writeScript "append-ldap-bind-pwd-in-${name}" ''
       #!${pkgs.stdenv.shell}