When you delete the main registration in an event, it delete all the additionnal registrations

We store main participant id in "registered_by_id" and currently it is "on delete cascade" hence additional participants are also deleted. So we should do following:
1. Change "registered_by_id" to on delete set null.
2. Modify delete participant screen

• If there are any additional participants associated with the participant that is been deleted then show below form

[ x ] Delete this participant record along with associated participant record(s).
[ ] Delete only participant record and associated contribution record.

3. If user selects first option, then modify postprocess to delete additional participants
if user selects second option, append following message after deleting

"The following participants no longer have an event fee recorded. You can edit their registration and record a replacement contribution by clicking the links below:"
Adam Bill ( Edit )
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Create an event with an online registration allowing multiple registrations.

Create a registration for John Doe (main participant) and put Jane Doe as a second participant.

Delete the participant John Doe, it automatically (without any warning) delete the participant Jane.

expected result: Jane is still in the list of participants.

X+