Display job definition schedule in a human-readable way

[JasonWeill]

Problem

The job definition schedule is stored in cron format (e.g., */5 * * * *) which most users will not recognize in text.

Proposed Solution

In the job definition list and details view, display the job definition schedule in a human-readable way (e.g., */5 * * * * as "every 5 minutes").

Additional context

All supported JupyterLab languages should be supported; the user's chosen locale should be used for display.

cron explanation libraries exist for multiple languages.

• Python: https://github.com/Salamek/cron-descriptor
• JavaScript: https://github.com/bradymholt/cRonstrue