Make the broadcast flow talk to the API

This commit removes the code the puts areas into the session and instead
creates and then updates a draft broadcast in the database.

This is so we can avoid session-related bugs, and potentially having a
large session when we start adding personalisation etc.

Once a broadcast is ready to go it is set to `broadcasting` straight
away with no approval. We’ll revisit this as we learn more about how
users might want to manage who can create and approve broadcasts.

The tests are a bit light in terms of checking what’s on the page, but
clicking through the pages is probably good enough for now.

tests/app/main/views/test_templates.py

@@ -696,6 +696,7 @@ def test_view_broadcast_template(
         ('Prepare broadcast', url_for(
             '.broadcast',
             service_id=SERVICE_ONE_ID,
+            template_id=fake_uuid,
         )),
         ('Edit', url_for(
             '.edit_service_template',