Split start and end date field in job model

[jezdez]

This is about the user experience of providing the date when scheduling new Spark jobs.

Right now when scheduling a new Spark job the "start date" and "end date" form fields both relate to the first and last date the job should run as well as the time of day when it's being executed. Since there is another UI element to define the run interval (daily, weekly, monthly) this is a confusing UX pattern.

Instead this should move the time value of the start and end date fields next to the interval field (in a new form field), so it's more in line with cron-like UX patterns.

The start date field indicates the first date of execution.

The end date field indicates the last date of execution and should explain that it will run then as well.