publish: Polish/refactor the publishing workflows. * Add a `--preview` option. * The `--list` option gives a nicer output, and prints warnings/errors for incorrect configuration. * Moved most of the `shell` code into a base class that's reusable. * Simplified the code to log publishing to a file. * Nicer output overall, with times.

{% extends "section-page.html" %}

{% block header %}
<header class="code">
    <h1>{{ page.title }}</h1>
</header>
{% endblock %}