Add permission to manage own registrations created by another account

If a user account has a registration made for them (by someone else using the "other account" setting), currently there's no way for the attendee account to manage that registration without having access to view/update/delete any registration. Patch forthcoming based on the work of @Mile23 in ✨ Allow user to manage his own registrations Closed: outdated (which is mainly about Views/lists) and their sandbox project https://www.drupal.org/sandbox/Mile23/1975698 → .