Update event entities to use Drupal datetime fields (#46)

* Update event entities to use Drupal datetime fields

* Event start and end dates now use UTC timezone

* Event start and end dates now are a single datetime range field

* Add dependency on module “Fullcalendar library”

* Add fullcalendar_library to makefile

* Don't fail build on php 7.2

* Change timezone to UTC

* Remove bat_fullcalendar.install

* Migrate event's dates on the new field

* Use short array syntax

* Event start and end dates now are a single datetime range field