Reminder settings update triggers notifications for past event instances

Problem/Motivation

The functionality for sending multiple reminders for an event (by updating the reminder settings in the event series, for example, initially scheduling a reminder for "1 month before" and later adding a new reminder for "1 day before") was introduced at #3283237.

While this feature is useful, an important aspect appears to have been overlooked: only future event instances should be marked for sending new reminders.

Currently, if a series contains both past and future instances, and the reminder settings are changed, reminder notifications are sent to registrants of past instances as well. This can cause confusion and potentially serious problems for users.

Steps to reproduce

1. Edit a series with some instances in the past and others in the future.
2. Change the reminder settings for the series.
3. Run cron.
4. Result: Registrants of past instances receive new reminder notifications, even though those events have already occurred.

Proposed resolution

1. Check the instance date.
2. Only re-enable reminders for future instances.
3. Skip any instances scheduled in the past.

Remaining tasks

Get the MR reviewed and merged.

User interface changes

None.

API changes

None.

Data model changes

None.