Travel stats to show number of conference stats

Closes: #167

templates/trip/stats.html

@@ -10,6 +10,7 @@
 <div class="container-fluid">
   <h1>Trip statistics</h1>
   <div>Trips: {{ count }}</div>
+  <div>Conferences: {{ conferences }}</div>
   <div>Total distance: {{ format_distance(total_distance) }}</div>
 
   {% for transport_type, distance in distances_by_transport_type %}
@@ -23,6 +24,7 @@
     {% set countries = year_stats.countries | sort(attribute="name") %}
     <h4>{{ year }}</h4>
     <div>Trips in {{ year }}: {{ year_stats.count }}</div>
+    <div>Conferences in {{ year }}: {{ year_stats.conferences }}</div>
     <div>{{ countries | count }} countries visited in {{ year }}:
       {% for c in countries %}
       <span class="text-nowrap">{{ c.flag }} {{ c.name }}</span>