nixos/mailman: httpd.services requires mailman-web in the systemd sense

When mailman-web restarts, it removes the generated "static" directory. This
breaks a currently running httpd process, which needs a re-start, too, to
obtain a new handle for the newly generated path.

nixos/modules/services/mail/mailman.nix

@@ -184,7 +184,7 @@ in {
     systemd.services.mailman-web = {
       description = "Init Postorius DB";
       before = [ "httpd.service" ];
-      wantedBy = [ "httpd.service" ];
+      requiredBy = [ "httpd.service" ];
       script = ''
         ${djangoExe}/bin/mailman-django-admin migrate --pythonpath ${cfg.webRoot} --settings settings
         rm -rf static