CronWorkflow schedule giving inconsistent experience

[dinever]

Summary

This issue contains several minor issues related to CronWorkflow:

AFIAK we don't support the second-level CronWorkflow schedule. Second-level CronWorkflow will be rejected when submitting. But the crontab translator seems to have no problem with it. This is an inconsistent user experience:

Furthermore, for an existing & valid CronWorkflow, the user can actually update it to a second-level CronWorkflow, and the schedule doesn't seem to work (Next schedule time says "xx seconds ago").

We should have consistent UX for this.

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Message from the maintainers:

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[alexec]

Changes needed:

1. Fix UpdateCronWorkflow (server/cronworkflow/cron_workflow_server.go) to invoke validate.ValidateCronWorkflow (like CreateCronWorkflow` does on line 35)
2. Remove Next Scheduled Time (ui/src/app/cron-workflows/components/cron-workflow-status-viewer.tsx#L27-35). I think it is better to remove this feature than it to be buggy because it is based on wrong information.
3. The UI validator resides in <ScheduleValidator/> (ui/src/app/cron-workflows/components/schedule-validator.tsx). However, this will always only be roughly correct - and we cannot predict when it might be wrong. Instead, we should add an <InfoIcon /> to let users know that seconds are not supported.