R1:91089acbe56b

Begin navigating the mess that is edits to recurring events

Summary:
Ref T11804. This puts us on a path toward some kind of reasonable behavior here.

Currently, cancelling recurring events makes approximately zero sense ever in any situation.

Instead, give users the choice to cancel just the instance, or all future events. This is similar to Calendar.app. (Google Calendar has a third option, "All Events", which I may implement).

When the user picks something, basically do that.

The particulars of "do…