vantage6 does not properly delete linked resources when deleting a collaboration

When a collaboration is deleted in vantage6, the linked resources (such as tasks from that collaboration) are not properly deleted.

This is partly to manage data properly, but also to prevent a potential (but unlikely) side-effect, 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, resulting in information disclosure.