Support time entries in FlatpickrDateBef filter plugin

Problem/Motivation

As discussed on BEF module's #3500976 issue → , BEF datepicker support days / months / years but not time entries. Since the jQuery UI datepicker they are using doesn't support time entries itself, and since Flatpickr does, it would be nice if the datetime_flatpickr_bef submodule could handle it.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes