Add flow for composing an alert without a template

We think that in some cases alerts will be composed in the moment, and
therefore making people first create a template is:
- not a good use of their time
- adding some conceptual complexity which they don’t need

This commit makes it possible to type some words and have them go
straight into the `content` field in the database.

In the future we might want to progressively enhance the radio buttons
so they show on the same page (like we do with the grey buttons on the
templates page).

tests/app/notify_client/test_broadcast_message_client.py

@@ -12,6 +12,8 @@ def test_create_broadcast_message(mocker):
     client.create_broadcast_message(
         service_id='12345',
         template_id='67890',
+        content=None,
+        reference=None,
     )
     mock_post.assert_called_once_with(
         '/service/12345/broadcast-message',