backoffice: fix SoS display ordering

closes #602

src/backoffice/templates/backoffice/event/components/list_content.html

@@ -36,8 +36,12 @@
       <tbody>
         {% for event in events %}
           <tr class="{% if not event.is_public %}text-muted{% endif %}">
-            <td>{{ event.schedule_start }}</td>
-            <td>{{ event.schedule_duration }}</td>
+            <td data-order="{{ event.schedule_start|date:"U" }}">
+              <abbr title="{{ event.schedule_start|date:"c" }}">{{ event.schedule_start|conference_time:conference }}</abbr>
+            </td>
+            <td data-order="{{ event.schedule_duration.total_seconds|default:"0" }}">
+              {{ event.schedule_duration|naturaltimespan }}
+            </td>
             <td>
               {% if user_list and event.kind == 'sos' %}
                 <a href="{% url 'backoffice:sos-edit' pk=event.pk %}">{{ event.name }}</a>

src/backoffice/templatetags/c3assemblies.py

@@ -45,8 +45,11 @@ def get_language_item(dictionary):
 
 
 @register.filter
-def naturaltimespan(x):
-    assert isinstance(x, timedelta), str.format('Expected a timedelta object: {0!r}', x)
+def naturaltimespan(x: timedelta) -> str or None:
+    if x is None:
+        return ''
+    if not isinstance(x, timedelta):
+        raise ValueError('Expected a timedelta object.')
 
     seconds = x.total_seconds()