_disabledButtonTemplate(label) {
    return html`
      <li>
        <button disabled>${this._prevNextIconTemplate(label)}</button>
      </li>
    `;
  }
Get disabled button template.
This method can be overridden to apply customized template in wrapper.
@param {String} label namespace label i.e. next or previous
@returns {TemplateResult} nav item template
@protected