Deleting old events with registrants

Sorry I've missed this in review as the status was still active, so I've moved it to Needs Review so I can pick it up soon.

Thanks for the patch, sorry for the late work on it. This issue seems to be causing a bunch of other issues, so I wanted to spend time and review properly.

I noticed a couple of issues with the patch, the biggest problem seemed to be that registrants could potentially be emailed multiple times for a series being deleted, which is not ideal. The patch was looping through the registrants and then looping through all the instances and sending emails for all future ones, whereas ideally it would just be sending one email for the whole series.

The underlying problem seems that we were only deleting registrants for future events, and not for past events too. So that has been addressed, but there may be sites where there are registrants hanging around from the past. So I put together a UI to allow administrators with the new permission 'administer orphaned events entities' to delete orphaned registrants (and also orphaned instances).

So in the case where an event instance or event series has been removed, but the registrant somehow survived the purge, the UI will highlight those and allow you to remove them all. The UIs for removing the orphaned instances and registrants are listed under Admin > Structure > Events

All code is in the MR now.

I submitted a patch against version 2.0.0-rc6 in https://www.drupal.org/project/recurring_events/issues/3322292

Bumping this up so I can get it merged soon. Thanks for the review and approval. Sorry about the delay.

Thanks for all involved, this has been merged into the latest dev branches.