Important: #90264 - Initialize datepicker JS in external file 

See forge#90264

Description 

The initialization of the datepicker has been moved into an external file residing in EXT:form/Resources/Public/JavaScript/Frontend/DatePicker.js. Some installations might restrict requesting public resources from /typo3/. Therefore, a new YAML configuration has been introduced:

TYPO3.CMS.Form.prototypes.standard.formElementsDefinition.DatePicker.properties.datePickerInitializationJavaScriptFile

That way, integrators are able to move the file to a different folder which is publicly accessible.