Reorganize template `base.html`.

Change layout from this:

    <body>
        <nav id='sitenav'>...</nav>
        <section id='shortcuts'>...</section>
        <nav id='breadcrumb'>...</nav>
        <section id='content'>...</section>
    </body>

To this:

    <body>
        <section id='shortcuts'>...</section>
        <nav id='sitenav'>...</nav>
        <nav id='breadcrumb'>...</nav>
        <section id='content'>...</section>
    </body>

The latter layout makes more intuitive sense (to the author).
Link "breadcrumb" navigation elements.

Create links in all breadcrumb navigation elements. For example, `item-id.html`
contains the following:

    {% block breadcrumb %}
        <li><a href='{% url 'elts.views.item' %}'>Item</a></li>
        <li><a href='{% url 'elts.views.item_id' item.id %}'>{{ item.id }}</a></li>
    {% endblock %}

This change means that the user finally has a full set of basic navigation
elements for the site. `nav#sitenav` provides navigation between each top-level
section of the site, and `nav#breadcrumb` provides navigation within each
section of the site.