Easy create: Create job schedule by minute, hour, day, weekday, week, or month

[JasonWeill]

Adds six additional options to schedule a job every minute, every hour at M minutes past the hour, every day at time T, every weekday at time T, every week at time T on day-of-week W, and every month at time T on day-of-month D.

Text inputs are validated. Changes are persisted in the cron schedule format.

Fixes #84, #98, #80.

Screen shots:

[github-actions]

👈 Launch a Binder on branch jweill-aws/jupyter-scheduler/easy-create

[andrii-i]

Tested this locally and difference in usability is night and day compared to previous scheduling versions.

Overall looks good, left some nit comments.