Dont allow reusing usernames

We have cases in which app developers forget to delete user data when a user is being deleted. This could be problematic as future users with the same username could then access this data.

This mitigates this risk a bit by not allowing reusing of usernames. That said, we still need to fix occurrences of these issues by deleting missed data.

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>