[Web] checkbox styling

Signed-off-by: Kristian Feldsam <feldsam@gmail.com>

data/web/templates/admin/tab-sys-mails.twig

@@ -60,7 +60,7 @@
         <div class="row mb-2">
           <div class="offset-sm-2 col-sm-10">
             <label>
-              <input type="checkbox" id="mass_disarm"> {{ lang.admin.activate_send }}
+              <input type="checkbox" class="form-check-input" id="mass_disarm"> {{ lang.admin.activate_send }}
             </label>
           </div>
         </div>