Switch Datepicker to Flatpickr

[ooduor]

Would like to suggest switch to Flatpicker (see https://flatpickr.js.org/examples/) as the date, datetime and also time picker. It seems to stable project to rely on has a lot of features that could be made optional or limited just to those three functionalities. I have already integrate it as custom deform widget elsewhere and would be happy to contribute if this is a welcome switch.

[stevepiercy]

@ooduor that is nice. Looks good on Chrome, Firefox, Safari, Brave, and Opera. On mobile the native datetime picker is nice. I have not tried IE/Edge.

I found a bug. Click on a datetime picker and note that the time selector appears only on the second click or by tapping the arrow keys enough times. That's not a deal breaker, but it should not do that. (EDIT: actually you have to click a date first, then the time appears.)

This is an improvement over the existing datetime pickers.

The fact that it does not depend on jQuery or Bootstrap is a good thing, I guess. The UI will not align exactly with Bootstrap, but it's close enough not to matter. It could even encourage folks to try Material-UI.