Synchronise/check synchronisation between event registrations and linked group

• Move all code related to managing the members of the linked group to an m2m_changed signal handler, and remove the handling code from the view
• Add a data check that triggers if registrations and group members get out of sync

changed title from Remove person from linked group if registration is deleted to Synchronise/check synchronisation between event registrations and linked group

changed the description

assigned to @nik

created branch 5-synchronise-check-synchronisation-between-event-registrations-and-linked-group to address this issue

mentioned in merge request !20 (merged)

marked the checklist item Add a data check that triggers if registrations and group members get out of sync as completed

marked the checklist item Move all code related to managing the members of the linked group to an m2m_changed signal handler, and remove the handling code from the view as completed

assigned to @debdolph and unassigned @nik

closed with merge request !20 (merged)

mentioned in commit e2ab1d96