Event Registration Reminder issues

Thanks your clear write up and for the report @phillamb168. I can reproduce the same, although it sounds like you found workarounds.

Here's my write up to help drafting a pull request. I should be able to draft something shortly.

1) Clicking Add instance on a series creates an instance with `reminder_date` empty
2) When `reminder_date` changes `reminder_sent` is emptied
3) If series dates changes but reminder options did not, instances are recreated without a `reminder_date` (It's expected Instances are recreated when series dates change)

I've attached a patch that address the issues and is pretty minimal. But recurring_events can be heavily relied on and by sites with lots of events traffic so it'd be helpful getting review from another maintainer.

Here a summary of the changes, which are within recurring_Events_registration:

1. Change hook `eventseries_insert` to `eventinstance_insert`
2. Change criteria in `eventseries_update` to simple yes/ no
3. Do not empty `reminder_sent`

I'll mark this as Needs review.