Update our attendee status based on the busyStatus

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
attendee status.

If we are not an attendee, we have to avoid adding an extra attendee
though.

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.

Differential Revision: https://git.kolab.org/D4481