Federation: orphaned links
Scenario:
a user /users/calum/
is deleted. Another user creates an account under the same username, inheriting the urlid. The backlinks system should ensure that all connecting resources are cleaned up, but there are other cases:
- the sender was unable to deliver the
Delete
activity to the receiver - the user is "deleted" because the server goes down and a malicious user buys the rights to the domain
- the server isn't configured properly and doesn't notify its peers of the deletion