Put 3rd level navigation in <ul> instead of <div>

[rc-davis]

Google Lighthouse complains when Isomer navigation menus contain 3rd level navigation, because the 3rd level is contained in a <div> tag. This was highlighted for the NCCS site.

The problem can be see on this test page. The navigation bar contains structures like the following:

<ul class="bp-menu-list">
    <li class="third-level-nav-header">
        <a aria-label="Page" class="third-level-nav-header is-active">Page
            <i aria-hidden="true" class="sgds-icon sgds-icon-chevron-up is-pulled-right is-size-4"/></a>
    </li>
    <div class="third-level-nav-div ">
        <li>
            <a class="third-level-nav-item padding--top--none has-text-secondary has-text-weight-bold" href="/collection/page/subpage">Subpage</a>
        </li>
    </div>
</ul>

We should fix this by putting 3rd level navigation inside another <ul> tag instead of a <div> tag.