Make "Create a new user" button on Admin users page visible

The list.html template extends admin.html which overrides
the 'page_header_block' (from layout.html) with:

    {% block page_header_block %}
        <section id="page-header" class="toolbar" style="height:5px;">
        </section>
    {% endblock %}

Thus there is no 'page_header' into which list.html can insert
the button. Move the button into the lower button toolbar so
that it appears.