Summary: Fixes T11804. This probably isn't perfect but seems to work fairly reasonably and not be as much of a weird nonsense mess like the old behavior was. When a user edits a recurring event, we ask them what they're trying to do. Then we more or less do that. Test Plan: - Edited an event in the middle of a series. - Edited the first event in a series. - Edited "just this" and "all future" events in various places in a series. - Edited normal events. - Cancelled various events. Reviewers: chad Reviewed By: chad Maniphest Tasks: T11804 Differential Revision: https://secure.phabricator.com/D16782