Add views exposed filter support

It seems that to use it in the normal exposed filter we would have the following options:
- Override and replace the Date filter handler class (since Drupal does not provide any filter widget plugins)
- Create a new handler for the datetime fields
- Create a generic configuration form which can inject the flatpickr library on any item based on a selector and with the generic config (might not work out)

The better exposed filter could work better as that module provides the mechanism to define a widget. Of course it also brings in the dependency, but as a sub module it could be fine.

The Better Exposed Filters module integrates with the date type fields, including the date popup. You can use this module to easily add an exposed filter with the date popup.