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.