Add a fix script to delete all accounts with invalid usernames. Also
change it so the owner-level user can delete accounts belonging to other
users.
Users who have logged in in the last year and who have a valid email
address will be given a one week warning. After that all accounts with
invalid names will be deleted. Anyone who has visited the site in the
last 6 months will have already seen a warning page that their name must
be changed to keep using the site.
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