Purge users who didn't verify their email within 24h

As of May 17 and using the previous 2-6 days as sample, around 12% of newly created users (on average, per day) do not verify their email. Manual review of the users shows they're all spammers, we don't need those users.

The use-case for a real user taking more than 24h to verify an email address after signup is niche, and where they really can't, well, they'll just have to sign up again.