[6881] replace datepicker with flatpicker

[goapunk]

Describe your changes
Replace the old datepicker with flatpickr and connect the flatpickr instances (if there are multiple ones, e.g. for module phases). Also adds a time-only datepicker to the time field.

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[goapunk]

@hom3mad3 this is more of a wip/concept to discuss, maybe you can have a look already before this is going in the wrong direction

[hom3mad3]

@goapunk where and how can i test this?

[goapunk]

@goapunk where and how can i test this?

in the dashboard on any module (phases setting), e.g. http://127.0.0.1:8003/dashboard/modules/participatory-budgeting-3-phase-3/phases/