Allow old broadcasts to be overridden

This is a temporary feature to make it easy to migrate the format
of the "areas" column and backfill extra data for it.

It's not possible to use this feature to update the status of an
old broadcast message, so the risk from this override is minimal.

tests/app/broadcast_message/test_rest.py

@@ -385,6 +385,10 @@ def test_update_broadcast_message_allows_edit_while_not_yet_live(admin_request,
     assert response['updated_at'] is not None
 
 
+@pytest.mark.parametrize('force_override', [
+    False,
+    pytest.param(True, marks=pytest.mark.xfail)
+])
 @pytest.mark.parametrize('status', [
     BroadcastStatusType.BROADCASTING,
     BroadcastStatusType.CANCELLED,
@@ -392,14 +396,17 @@ def test_update_broadcast_message_allows_edit_while_not_yet_live(admin_request,
     BroadcastStatusType.TECHNICAL_FAILURE,
 ])
 def test_update_broadcast_message_doesnt_allow_edits_after_broadcast_goes_live(
-    admin_request, sample_broadcast_service, status
+    admin_request,
+    force_override,
+    sample_broadcast_service,
+    status
 ):
     t = create_template(sample_broadcast_service, BROADCAST_TYPE)
     bm = create_broadcast_message(t, areas=['manchester'], status=status)
 
     response = admin_request.post(
         'broadcast_message.update_broadcast_message',
-        _data={'areas': ['london', 'glasgow']},
+        _data={'areas': ['london', 'glasgow'], 'force_override': force_override},
         service_id=t.service_id,
         broadcast_message_id=bm.id,
         _expected_status=400