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)

tests/app/dao/test_broadcast_message_dao.py

@@ -122,10 +122,10 @@ def test_dao_get_all_broadcast_messages(sample_broadcast_service):
     assert broadcast_messages == [
         (
             broadcast_message_2.id, None, 'severe', 'Dear Sir/Madam, Hello. Yours Truly, The Government.',
-            {'areas': [], 'simple_polygons': []}, 'broadcasting', datetime(2021, 6, 20, 12, 0),
+            {'areas': [], 'ids': [], 'simple_polygons': []}, 'broadcasting', datetime(2021, 6, 20, 12, 0),
             None, None, None),
         (
             broadcast_message_1.id, None, 'severe', 'Dear Sir/Madam, Hello. Yours Truly, The Government.',
-            {'areas': [], 'simple_polygons': []}, 'cancelled', datetime(2021, 6, 15, 12, 0),
+            {'areas': [], 'ids': [], 'simple_polygons': []}, 'cancelled', datetime(2021, 6, 15, 12, 0),
             None, None, None)
     ]