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 that" are messy. We have to split the series into two different series, stop the first series early, then edit the second series. Then we need to update any concrete events that are now part of the second series.

This code will get less junk in the next couple of diffs (I hope?) since I need to make it apply to edits, too, but this was a little easier to get started with.

Test Plan:
Cancelled an instance of an event; cancelled "All future events".

Both of them more or less worked in a reasonble way.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11804

Differential Revision: https://secure.phabricator.com/D16778