Count only verified users on /statistics page

With bot attacks it’s easy to create a large number of users that seemingly let the site grow. Statistics should show a more trustworthy number.

• in Coocook::Schema->statistics()
  • rename the key users to verified_users
  • reduce the User resultset to verified users before calling count()
• adjust the hash key name in the users <div> in root/templates/statistics.tt
• adjust the hash key name the statistics subtest in t/schema.t