Update page titles.

Make the titles of several pages more consistent.

The page titles, as they stand, are currently awkward and rather useless. It
might be better if, for example, the very top header on the page is used as a
breakcrumb nav section, linking to each portion of the current URL (RESTful URL
design makes this possible). The more friendly and informative page title could
then reside with the remainder of the content. For example, this might go at the
very top of a page:

    <ol>
        <li><a>Item</a></li>
        <li><a>{{ item.id }}</a></li>
        <li><a>Update Form</a></li>
    </ol>

And this would go at the top of the content just following it:

    <h1>{{ item.name }}</h1>
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.