new broadcast_message table. contains a whole bunch of timestamps, a
couple of user ids for who created and who approved, some
personalisation and a template id/version. areas is a jsonb field - it
is expected that this will be an array, for example `["england",
"wales"]`
intended valid state transitions are as follows, from an initial status
of draft
* draft -> pending-approval, rejected
* pending-approval -> broadcasting, rejected, (draft maybe)
* broadcasting -> completed, cancelled
rejected, completed, cancelled and technical-failure are terminating
states.
also copy across the enum switching code from migration 0060 (adding
letter type).
