Travel stats: distance and price by year and travel type

Closes: #155

templates/trip/list.html

@@ -48,11 +48,12 @@
 {% endblock %}
 
 {% macro section(heading, item_list) %}
-  {% if item_list %}
     {% set items = item_list | list %}
     <div class="heading"><h2>{{ heading }}</h2></div>
+    <p><a href="{{ url_for("trip_stats") }}">Trip statistics</a></p>
     <p>{{ items | count }} trips</p>
 
+  {% if item_list %}
     <div>Total distance: {{ format_distance(total_distance) }}</div>
 
     {% for transport_type, distance in distances_by_transport_type %}