Add "breadcrumb" navigation element.

The semantics of `base.html` have changed. The layout of that file now has this
top-level design:

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

nav#sitenav allows navigation between top-level sections of the site, such as
/item and /tag. nav#breadcrumb allows navigation within a single section of the
site, such as /item/42/update-form and /item/42. section#shortcuts contains
tools such as the log out button and "priority categories" (links to groups of
items, filtered by their tags). section#content contains the bulk of content
unique to each page.

This commit removes the <div> introduced in the previous commit, and the
contents of each template inheriting from `base.html` are much more natural.