Fix cron edit traceback.

What does this MR do and why?

Fixes issue on post with adding users when no email is included (this also affected editing users).

Fixes issue with schedules allowing days to be more than 30.

Fixes issue with schedules when trying to edit a cron task.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• Have you checked this doesn't interfere/conflict/duplicate someone elses work?
• Have you fully tested your changes?
• Have you resolved any lint issues?
• Have you assigned a reviewer?
• Have you applied correct labels?