Convert ManageEvent Registration Tab to datepicker

[mattwire]

Overview

Convert form to use datepicker:
Manage Event - Registration.

Partial from #12973

Before

Using old jcalendar.

After

Using "new" datepicker.

[civibot]

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[eileenmcnaughton]

I tested adding editing & deleting an event with these fields (online registration start & end date) & they worked fine. I also tested the rule.

[seamuslee001]

Merging as per the tag