Add definition for event statuses

[PGijsbers]

The Event Status taxonomy currently defines four statuses, "upcoming" and "planned" seem the same to me without further clarification. Certainly easily confused. The only thing I can think of is that one indicates a date has been set whereas for the other it's merely announced without dates? In any case, clarification should be added.

[antoniskappa]

This value set is probably coming from the previous version of the platform. We will look into that and update the taxonomy accordingly.