[Web] fix setting unchecked checkboxes

data/web/templates/modals/mailbox.twig

@@ -381,6 +381,12 @@
       </div>
       <div class="modal-body">
         <form class="form-horizontal" data-cached-form="true" data-id="add_domain" role="form">
+          <input type="hidden" value="0" name="gal">
+          <input type="hidden" value="0" name="active">
+          <input type="hidden" value="0" name="backupmx">
+          <input type="hidden" value="0" name="relay_all_recipients">
+          <input type="hidden" value="0" name="relay_unknown_only">
+
           <div class="row mb-2">
             <label class="control-label col-sm-2 text-sm-end text-sm-end" for="domain">{{ lang.add.domain }}</label>
             <div class="col-sm-10">