Resolve #439 ability to cancel individual event date

[NoRePercussions]

Adds a cancelled option to eventdates, and makes cancelled dates mirror cancelled event styles in the event list and when editing an event.

As of 2a092ff8 a date marked cancelled can still be billed for if marked show billable or similar. This does not reflect how events work (cannot bill for cancelled events even if they are marked billable), but may be necessary for edge cases such as an event getting cancelled after an early setup but before the show.

[DaAwesomeP]

I'm going to close this since I think you agree that this is superseded by #536.