Add countries visited to trip stats page

Closes: #160

templates/trip/stats.html

@@ -20,8 +20,14 @@
   {% endfor %}
 
   {% for year, year_stats in yearly_stats | dictsort %}
+    {% set countries = year_stats.countries | sort(attribute="name") %}
     <h4>{{ year }}</h4>
     <div>Trips in {{ year }}: {{ year_stats.count }}</div>
+    <div>{{ countries | count }} countries visited in {{ year }}:
+      {% for c in countries %}
+      {{ c.flag }} {{ c.name }}
+      {% endfor %}
+      </div>
     <div>Total distance in {{ year}}: {{ format_distance(year_stats.total_distance) }}</div>
     {% for transport_type, distance in year_stats.distances_by_transport_type.items() %}
       <div>