It seems, for outlook the busystate is the attendee state.
This means we shouldn't set STATUS based on it, but we should update our
If we are not an attendee, we have to avoid adding an extra attendee
To get consistent behaviour in both directions, we also set the
busyStatus based on the attendee status, which makes sure that we get
the same result no matter if the webclient or outlook is used to set to
tentative or accepted.