Anonymize project applicants and participants on creative delete

[simonbaese]

The user references in the project fields participants and applicants should be anonymized on user deletion. At the moment, fetching user references for non-existent users will result in an empty "response". Therefore, it is not a problem to leave the references for now. Replacing the reference with the anonymous user has the benefit that we can deliver it to the front-end and display that there was a relation.

Note that the related field participant_tasks is a little quirky at the moment. Therefore, its order needs to be respected and eventually updated as well! One should consider refactoring it - but that is a little harder (proper field type).

See logbook_user_delete for some example queries.

[simonbaese]

This will also apply to project results and their respective comments. But there is a refactoring planned for project comments.