Display playtime graph on session list

Fixes #29

src/web/tracker/templates/list_sessions.html

@@ -5,11 +5,14 @@
 {% block content %}
         <div class="text-center text-xl mb-4 dark:text-slate-400">
             {% if dataset.count >= 2 %}
+            <img src="data:image/svg+xml;base64,{{ chart|safe }}" class="mx-auto mb-5" />
             <a href="{% url 'start_session' dataset.last.purchase.id %}">
                 <button type="button" title="Track last tracked" class="py-1 px-2 bg-green-600 hover:bg-green-700 focus:ring-green-500 focus:ring-offset-blue-200 text-white transition ease-in duration-200 text-center text-base font-semibold shadow-md focus:outline-none focus:ring-2 focus:ring-offset-2 rounded-lg ">
                     New session of {{ dataset.last.purchase }}
                 </button>
             </a>
+            {% else %}
+            Playtime chart will be displayed when there are 2 or more sessions.
             {% endif %}
             {% if purchase %}
             <h1>Listing sessions only for purchase "{{ purchase }}"</h1>