Registration now uses the standard entity handler pattern for host entities

Summary

Registration now uses the standard entity handler pattern.

The entity type id specified should be that of the entity to be wrapped as a host, not 'registration'. The handler should be 'registration_host_entity' not 'host_entity'.

This allows modules to provide alternative host entity handlers for entity types.

Before

$handler = \Drupal::entityTypeManager()->getHandler('registration', 'host_entity');
$host_entity = $handler->createHostEntity($entity);

After

$handler = \Drupal::entityTypeManager()->getHandler($entity->getEntityTypeId(), 'registration_host_entity');
$host_entity = $handler->createHostEntity($entity);