Support optional end date module

The optional end date module (https://www.drupal.org/project/optional_end_date) covers support for an optional end date on date range inputs. Now when you install this module together with Datetime Flatpickr, the optional end date configuration is being ignored, because they both use different field widgets.
To cover our needs to be able to use both modules together, I've added additional code to the Flatpickr datetime range picker separate inputs. This may be useful for others too, regarding the fact that there is a core issue about optional end date support.