Convert Event Participant Registration Date to datepicker

[mattwire]

Overview

Convert form to use datepicker:

• Register Participant - Registration Date

Partial from #12973

Before

Using old jcalendar.

After

Using "new" datepicker.

[civibot]

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[colemanw]

I can confirm this works in the UI, but CRM_Event_Form_ParticipantTest tests will need to be updated as this is causing failures.

[seamuslee001]

@mattwire should this still be WIP?

[eileenmcnaughton]

49 instances of jcalendar left...

[seamuslee001]

Removed WIP as it seems Matt has fixed the problem with the tests, Based on Coleman's review and the tag i'm going to merge this

[mattwire]

@seamuslee001 Thanks, think I was waiting for the tests to pass, but seeing as they did 👍