internal: Improve handling of taxonomy term slugification. This paves the way to bring slugification options like transliteration to PieCrust. This change mostly makes sure we use one-way slugification, which means, for serving/previewing, we need to slugify each page's terms to compare to the stuff captured from the URL. It also simplifies things a bit in the code.

<ul class="nav navbar-nav navbar-right">
    <li><a href="{{ pcurl('getting-started') }}">Getting Started</a></li>
    <li><a href="{{ pcurl('docs') }}">Documentation</a></li>
    <li><a href="{{ pcurl('api') }}">Code</a></li>
    <li><a href="{{ pcurl('support') }}">Support</a></li>
</ul>