Event status ActivityPub representation

Currently we use the following that matches iCalendar's STATUS property.

"ical:status": "TENTATIVE"|"CONFIRMED"|"CANCELLED"

Schema.org has an eventStatus property on Events that has the https://schema.org/EventStatusType, but it lacks a value to express that the event is tentative.

According to this document from 2013, page 3, if we assume events to work the same way as actions, an event with no startTime is assumed tentative. However, it would be nice to have an event as tentative and still have a starting date-time.

Therefore let's keep our ical:status for now. We can also implement schema.org's eventStatus for EventCancelled and EventScheduled values later.