Delete dates and times

[sander]

Not sure yet whether we really want this. It might be confusing invitees when options disappear, and entered availability data may be lost.

Maybe only make dates deletable when it hasn't been replied on yet?

A use case suggested by email was when accidentally the wrong dates are added. An undo button might work for this. (Suggestion: don't actually add the date for ~10 seconds, and display Undo during these.)

New use case suggested by email:

1. The appointment is created with a lot of possible dates.
2. Many of the dates appear not to be an option (probably due to important people not being available on those dates).
3. The initiator wants to remove those dates, in order to keep an overview and more easily make a decision.

I think this should not be about removing dates, but about hiding them from the overview. If they are re-added by any user, they can be prefilled with the previously entered availability data.