Proper support for meta-listing pages: - Added support for URL endpoints. - Moved all category-related stuff to generic meta handling. - Generic meta-listing pages in the front end. Miscellaneous fixes: - Remove previous "ready" items before caching new ones. - Only un-"ready" pages that have includes and queries. - Better error handling in `DatabasePage` auto-update. - Make sure pages have extended data cached before running queries.

<article>
    <header>
        <h1>{{meta_value}} <span class="decorator">{{meta_query}}</span></h1>
    </header>
    <section>
        {{content}}
    </section>
    <footer>
    </footer>
</article>