Retain participations for deleted users

[jeriox]

When users are not required anymore (e.g. because the volunteer left the organisation), they are deleted. Currently their participations are deleted along with them. For historical context of events, it would be good to retain a participation for them. We could convert the local participations to placeholder participations. Keeping the name of the user is probably not a good idea (GDPR), so smth like "Deleted User (RS)" could work