cubicweb-conference #856249 add a box "registration is open" [open]

we have a box "abstract submission is open", but we need a box "conference registration is open" that will lead to creating a new account and register for the conference.

There are more information in a registration than just "CWUser registered_for Conference": buy a tee-shirt, participate to the dinner, vegetarian food or not, etc, etc.

We need to have a look at EuroPython registration forms and make sure we do not forget anything.

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