Deleting a user should anonymize all entries in order to be GDPR conform

When deleting a user, all its associated wiki changes / comments / etc. needs to be anyonmized according to DSGVO / GDPR

Steps to reproduce:

1. A creates a wiki page W1
2. B creates a wiki page W2
3. A changes the wiki page W2
4. A. adds a comment to wiki page W3
5. Admin deletes user A

After step 4: W1, W2 and W3 will somehow have a association to user A (new page, edited page, comment)
After step 5: A is still in the history of W1, W2 and W3

Expected: the user should be completely anonymized according to DSGVO / GDPR

See discussion at: https://forum.xwiki.org/t/deleting-a-user-should-anonymize-all-entries/7473

This would be a possible extension of XWIKI-12142