Committing more progress on client-side validation of cron job form

* Added custom code for displaying client-side errors since most of the time, validation errors can't be displayed right after the element being validated. Migrated both simple and advanced form to use this system, which allows validation messages to be displayed in any of the parents of the element being validated.
* Worked around a limitation of the jQuery Validation plugin which causes disabled fields to retain already displayed validation messages on subsequent form validations (jzaefferer/jquery-validation#224).
* Fixed additional settings for "Repeat" options not being properly validated.