Check completeness of event before making public

[matthijskooijman]

The system should prevent making an incomplete event public. In particular things like a missing website URL or e-mail address, since those are used in notifications (but they are not required, since they might be inherited from the series).

A missing registration open date might also be checked for, though it is conceivable that you want an event public without its registration scheduled yet (OTOH, that's probably almost always a mistake?).