Split the application into two flows - this is part one (email).

Please don't merge this yet. Despite all tests passing, I'd like us to play with this on staging first.
I wonder if these might be better served as `prisoner.prison_allows_instant_booking?` and `process_as_email?` on either the visit or the session, in the long term? (I may well be misunderstanding how you intend for this to be consumed, though…)