Templates:
- heartbeat_initial
- heartbeat_reminder
- vote_initial
- vote_reminder
- recipient_unlock

Examples:
  php scripts/preview_email_templates.php heartbeat_initial > /tmp/hb.html
  php scripts/preview_email_templates.php vote_initial > /tmp/vote.html
  php scripts/preview_email_templates.php recipient_unlock > /tmp/recipient.html