[SSG] Move archives, subscribe, donate into directories.

Allow templates to be relative using {{ relative }} which can be set by
caller.

archives.html

@@ -2,6 +2,6 @@
 {% block left_content %}
 <ul class="archives">
 {% for episode in episodes %}
-	<li><a href="{{ episode.slug }}">{{ episode.title }}</a></li>
+	<li><a href="../{{ episode.slug }}">{{ episode.title }}</a></li>
 {% endfor %}
 {% endblock %}