Add a link to latest publication on homepage

Note: the CSS is utterly horrible. Please change this asap.
Close #5

mainsite/static/css/site-base.css

@@ -135,3 +135,10 @@ body {
   color: #505050;
   font-style: italic;
 }
+
+.last-publication {
+  font-size: 2em;
+  text-align: center;
+  padding: 30px;
+  font-weight: bold;
+}

mainsite/templates/mainsite/homepage.html

@@ -5,6 +5,13 @@
 {% block content %}
 {% get_solo 'mainsite.SiteConfiguration' as site_config %}
 
+<div class="last-publication">
+  <p>
+    Demandez la
+    <a href={% url "latestPublication" %}>dernière édition</a> !
+  </p>
+</div>
+
 {{ site_config.homepageText | markdownify }}
 
 {% endblock content %}