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.

• 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.