Update archives to display the date once all articles on the date.

The old method printed the date for every article even if they had the
same publish date. This update prints the date once for all articles
published on that date.

templates/archives.html

@@ -8,8 +8,12 @@
   </header>
   <div>
     <dl>
+      {% set previous_date = False %}
       {% for article in dates %}
+        {% if article.locale_date != previous_date: %}
+          {% set previous_date = article.locale_date %}
       <dt>{{ article.locale_date }}</dt>
+        {% endif %}
       <dd><a href="{{ SITEURL }}/{{ article.url }}">{{ article.title }}</a></dd>
       {% endfor %}
     </dl>