Edit all future events overrides individual event edits

Bug: change to all recurring events overrides previous changes to individual events

Changing the duration of all upcoming instances of a recurring event via "edit all future events" wipes out previous per-instance edits instead of only applying the changed property.

Steps: a weekly recurring event was created for a few months until an end date; some instances had their title edited and a few were deleted entirely (each edit made with "edit only this event"). Then the duration of all upcoming events was changed from 3 hours to 1 hour by editing one instance and selecting "edit all future events". Even though only the time was meant to change, this deleted all previous changes made to individual events, and the deleted instances reappeared in the calendar.

Expected: "edit all future events" only affects the properties that were actually edited (in this case the time) and leaves everything else (like title and description) the way it is for every individual event.