Replace flags with per-type state machines

[SamLR]

The CfP has many stages/modes (open, closed, invite only, edit only) and these often want to be managed independently for different types (e.g. talks & workshops often close earlier than performances)

Currently these are all managed by a variety of flags which have to be set and managed individually. Replacing these with state machines should mean it's easier to express the progression and managing each state machine per type should decouple their behaviour

[marksteward]

Also include whether a type is visible on the line-up. I think this should be a new table, and respected by schedule.ics, etc.

[Jonty]

This also needs to include the API - we just had content visible in the API that isn't yet visible on the line-up because that is still managed by hacking the code.