Remove old uncofirmed user records

[matschaffer]

We had a recent case where we were getting spam user registrations.

Devise ensures we only allow confirmations for 3 days https://github.com/Safecast/safecastapi/blob/master/config/initializers/devise.rb#L142

So any unconfirmed records older than 3 days are basically useless. Might be good to delete them on a schedule.