s="navbar-nav">
            <li class="nav-item">
                <a class="nav-link" href="#" th:href="@{/index}">Home</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#" th:href="@{/addcourse}">Add Course</a>
            </li>
        </ul>
        <!--ul class="navbar-nav ml-auto">
            <li class="nav-item">
                <form th:action="@{/logout}" method="post">
                    <button class="btn btn-danger" type="submit">logout</button>
  