Add some views for selecting broadcast areas

These are just so we have some pages to click through for now. They
don’t use real templates, or any of the broadcast stuff from the
database.

But I think it’s useful to get some skeleton pages in first so that we
can see the map etc working in production, then build on that, without
having to do it all in one mega PR.

For that reason there are two short term things I’ve done in this commit
which should be revisited soon:
- no tests for the endpoints
- data about which areas are selected is stored in the session

app/main/__init__.py

@@ -7,6 +7,7 @@ from app.main.views import (  # noqa isort:skip
     add_service,
     agreement,
     api_keys,
+    broadcast,
     choose_account,
     code_not_received,
     conversation,