Sort broadcasts by start time

For emails and text messages we sort by the time the user (or API) sent
them.

This makes sense for broadcasts too, since most users will receive the
alert within seconds of it being broadcast.

For alerts that haven’t started yet we can sort by `updated_at`, which
is when the user preparing the broadcast submitted it for approval.

tests/__init__.py

@@ -649,10 +649,12 @@ def broadcast_message_json(
     starts_at=None,
     finishes_at=None,
     cancelled_at=None,
+    updated_at=None,
     approved_by_id=None,
     cancelled_by_id=None,
     areas=None,
     content=None,
+    template_name='Example template',
 ):
     return {
         'id': id_,
@@ -661,7 +663,7 @@ def broadcast_message_json(
 
         'template_id': template_id,
         'template_version': 123,
-        'template_name': 'Example template',
+        'template_name': template_name,
         'content': content or 'This is a test',
 
         'personalisation': {},
@@ -677,7 +679,7 @@ def broadcast_message_json(
         'created_at': None,
         'approved_at': None,
         'cancelled_at': cancelled_at,
-        'updated_at': None,
+        'updated_at': updated_at,
 
         'created_by_id': created_by_id,
         'approved_by_id': approved_by_id,