Drupal8: drupal_set_message is deprecated (or: Event Cart messages display HTML)
When using the CiviCRM Event Cart in Drupal8, some status messages display HTML to the user.
This PR fixes two things:
- It uses
\Drupal\Core\Render\Markup::createto handle markup before displaying it.
- It uses the Messenger service, because drupal_set_message is deprecated since Drupal 8.5.x (2017) https://www.drupal.org/node/2774931
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