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).

tests/app/celery/test_broadcast_message_tasks.py

@@ -156,6 +156,44 @@ def test_send_broadcast_provider_message_sends_data_correctly(
     )
 
 
+@freeze_time('2020-08-01 12:00')
+def test_send_broadcast_provider_message_sends_data_correctly_when_broadcast_message_has_no_template(
+    mocker, sample_service,
+):
+    broadcast_message = create_broadcast_message(
+        service=sample_service,
+        template=None,
+        content='this is an emergency broadcast message',
+        areas={
+            'areas': ['london', 'glasgow'],
+            'simple_polygons': [
+                [[50.12, 1.2], [50.13, 1.2], [50.14, 1.21]],
+                [[-4.53, 55.72], [-3.88, 55.72], [-3.88, 55.96], [-4.53, 55.96]],
+            ],
+        },
+        status=BroadcastStatusType.BROADCASTING
+    )
+    event = create_broadcast_event(broadcast_message)
+
+    mock_create_broadcast = mocker.patch(
+        f'app.clients.cbc_proxy.CBCProxyEE.create_and_send_broadcast',
+    )
+
+    send_broadcast_provider_message(provider='ee', broadcast_event_id=str(event.id))
+
+    broadcast_provider_message = event.get_provider_message('ee')
+
+    mock_create_broadcast.assert_called_once_with(
+        identifier=str(broadcast_provider_message.id),
+        message_number=mocker.ANY,
+        headline='GOV.UK Notify Broadcast',
+        description='this is an emergency broadcast message',
+        areas=mocker.ANY,
+        sent=mocker.ANY,
+        expires=mocker.ANY,
+    )
+
+
 @pytest.mark.parametrize('provider,provider_capitalised', [
     ['ee', 'EE'],
     ['vodafone', 'Vodafone'],