Allow users to define custom cron schedules for the tracker jobs

[azasypkin]

Summary

Currently, users can only schedule content and resource tracker jobs on an hourly, daily, weekly, or monthly interval. Users should have the ability to specify a custom cron expression for the schedule as well (within reasonable boundaries).

See https://docs.oracle.com/cd/E12058_01/doc/doc.1014/e12030/cron_expressions.htm, zslayton/cron#37 (comment) and https://www.freeformatter.com/cron-expression-generator-quartz.html

[azasypkin]

Handled via acbd82a and secutils-dev/secutils-webui@ef77035. See documentation at https://secutils.dev/docs/guides/web_scraping/content#annex-custom-cron-schedules