[IMP] event: replace state by kanban stage

Purpose
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Remove the state on the event and add a stage.
So, we can have more control on the event flow, and we will have less constrains.

Website event
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Before
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People can register for an event if "seats are available" and if the state is "confirmed".

After
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People can register for an event if `event_registrations_open` is True,
- Event: seats are available and the event is not finished
- Event sale:  One or more ticket has `sale_available` set to True

Task #2088538