Implement sorting

Shows the broadcasts with the longest time still to live at the top. At
the moment this will be the same as the newest broadcasts, so we may
want to revisit this sort order when we have broadcasts of variable
duration.

For no-longer-live broadcasts we show the most-recently-finished at the
top, whether it finished naturally or was cancelled.

app/models/broadcast_message.py

@@ -36,6 +36,13 @@ class BroadcastMessage(JSONModel):
 
     libraries = broadcast_area_libraries
 
+    def __lt__(self, other):
+        return (
+            self.cancelled_at or self.finishes_at
+        ) < (
+            other.cancelled_at or other.finishes_at
+        )
+
     @classmethod
     def create(cls, *, service_id, template_id):
         return cls(broadcast_message_api_client.create_broadcast_message(

app/templates/views/broadcast/dashboard.html

@@ -5,7 +5,7 @@
 {% macro broadcast_table(broadcasts, empty_message) %}
   <div class='dashboard-table ajax-block-container'>
     {% call(item, row_number) list_table(
-      broadcasts,
+      broadcasts|sort|reverse|list,
       caption="Live broadcasts",
       caption_visible=False,
       empty_message=empty_message,

tests/app/main/views/test_broadcast.py

@@ -83,8 +83,8 @@ def test_broadcast_dashboard(
     assert [
         normalize_spaces(row.text) for row in page.select('table')[1].select('tbody tr')
     ] == [
-        'Example template To England and Scotland Finished yesterday at 8:20pm',
         'Example template To England and Scotland Stopped 10 February at 2:20am',
+        'Example template To England and Scotland Finished yesterday at 8:20pm',
     ]