add content to broadcast_message and make template fields nullable

we want to be able to create broadcast messages without templates. To
start with, these will come from the API, but in future we may want to
let people create via the admin interface without creating a template
too.

populate a non-nullable content field with the values supplied via the
template (or supplied directly if via api).

app/broadcast_message/rest.py

@@ -101,16 +101,20 @@ def create_broadcast_message(service_id):
     user = get_user_by_id(data['created_by'])
     template = dao_get_template_by_id_and_service_id(data['template_id'], data['service_id'])
 
+    personalisation = data.get('personalisation', {})
     broadcast_message = BroadcastMessage(
         service_id=service.id,
         template_id=template.id,
         template_version=template.version,
-        personalisation=data.get('personalisation', {}),
+        personalisation=personalisation,
         areas={"areas": data.get("areas", []), "simple_polygons": data.get("simple_polygons", [])},
         status=BroadcastStatusType.DRAFT,
         starts_at=_parse_nullable_datetime(data.get('starts_at')),
         finishes_at=_parse_nullable_datetime(data.get('finishes_at')),
         created_by_id=user.id,
+        content=template._as_utils_template_with_personalisation(
+            personalisation
+        ).content_with_placeholders_filled_in,
     )
 
     dao_save_object(broadcast_message)
@@ -185,16 +189,11 @@ def _create_broadcast_event(broadcast_message):
     else:
         transmitted_finishes_at = broadcast_message.finishes_at
 
-    template = broadcast_message.template._as_utils_template_with_personalisation(
-        # Broadcast events don’t support personalisation yet
-        values={}
-    )
-
     event = BroadcastEvent(
         service=broadcast_message.service,
         broadcast_message=broadcast_message,
         message_type=msg_types[broadcast_message.status],
-        transmitted_content={"body": str(template)},
+        transmitted_content={"body": broadcast_message.content},
         transmitted_areas=broadcast_message.areas,
         # TODO: Probably move this somewhere more standalone too and imply that it shouldn't change. Should it include
         # a service based identifier too? eg "flood-warnings@notifications.service.gov.uk" or similar