oliverdavies.uk/source/_includes/nav.html.twig

<nav class="navbar navbar-inverse navbar-fixed-top">
    <div class="container">
        <div class="navbar-header">
            <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
                <span class="sr-only">Toggle navigation</span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>
            <a class="navbar-brand" href="{{ site.url }}/">{{ site.title }}</a>
        </div>

        <div id="navbar" class="collapse navbar-collapse" role="navigation">
            <ul class="nav navbar-nav">
                {% for id, link in site.main_menu %}
                    <li class="{{ page.nav == id ? 'active' }}">
                        <a href="{{ site.url }}{{ link.href }}">{{ link.title }}</a>
                    </li>
                {% endfor %}
                <li>
                    <a href="mailto:{{ site.email }}?subject=Email%20from%20{{ site.short_url }}">Contact <i class="fa fa-envelope-o"></i></a>
                </li>
            </ul>
        </div>{# .nav-collapse #}
    </div>
</nav>