Event status ActivityPub representation
Currently we use the following that matches [iCalendar's `STATUS` property](https://www.kanzaki.com/docs/ical/status.html). ``` "ical:status": "TENTATIVE"|"CONFIRMED"|"CANCELLED" ``` Schema.org has an `eventStatus` property on Events that has the [https://schema.org/EventStatusType](https://schema.org/EventStatusType), but it lacks a value to express that the event is tentative. According to [this document from 2013](https://www.w3.org/wiki/images/3/38/ActionsinSchema.org2013-05-11.pdf), 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.
issue