Deleting a collaboration should also delete linked resources

vantage6 is privacy preserving federated learning infrastructure. When a collaboration is deleted, the linked resources (such as tasks from that collaboration) should be deleted. This is partly to manage data properly, but also to prevent a potential (but unlikely) side-effect that affects versions prior to 4.0.0, where if a collaboration with id=10 is deleted, and subsequently a new collaboration is created with id=10, the authenticated users in that collaboration could potentially see results of the deleted collaboration in some cases. Version 4.0.0 contains a patch for this issue. There are no known workarounds.