Implement categories

This currently works only somewhat, since the URLs are too relative. Not
related to RELATIVE_URLS setting, though.

test-theme/templates/category.html

@@ -1,6 +1,11 @@
 {% extends 'base.html' %}
-{% set page_title = "TODO" %}
+{% set page_title = "Category: " + category.name %}
 {% block content %}
-{# TODO! #}
+<h1>Category: {{ category.name }}</h1>
+<ul>
+{% for article in articles %}
+<li><a href="{{ article.url }}">{{ article.title }}</a>
+{% endfor %}
+</ul>
 {% endblock content %}