Start dual running with "areas" and (area) "ids"

This is necessary until:

- The Admin app is using the new "areas(_2)" format to store and
retrieve data.

- We've migrated all existing broadcast messages to use the new
format.

Note that "areas" / "ids" isn't actually used for anything except
printing out the PagerDuty message - it's not sent to the proxy [1].

[1]: 6edc6c70aa/app/celery/broadcast_message_tasks.py (L190-L193)

app/celery/broadcast_message_tasks.py

@@ -135,7 +135,7 @@ def send_broadcast_event(broadcast_event_id):
             f'https://www.notifications.service.gov.uk/services/{broadcast_message.service_id}/current-alerts/{broadcast_message.id}',  # noqa
             '',
             f'This broacast has been sent on channel {broadcast_message.service.broadcast_channel}.',
-            f'This broadcast is targeted at areas {broadcast_message.areas.get("areas")}.',
+            f'This broadcast is targeted at areas {broadcast_message.areas.get("ids", broadcast_message.areas.get("areas"))}.',  # noqa
             ''
             f'This broadcast\'s content starts "{broadcast_message.content[:100]}"'
             '',