Retire the 24h welcome reminder email

We were sending out a welcome reminder email to new users 24 hours after the initial welcome email.

This caused a problem when admins resend the welcome email. If they resend the original welcome instead of the welcome reminder, the setup link in the email doesn’t work because the link in the later reminder email invalidates older links.

The quest progress emails also serve as reminders for users to set up their account, so we retired the 24h welcome reminder email.