Reset engagement score for inactive subscribers

If someone hasn't received at least 3 emails in the last year, we can't
trust that their level of engagement would be the same now as it was
over a year ago.

MAILPOET-5410

mailpoet/lib/Statistics/StatisticsOpensRepository.php

@@ -33,6 +33,7 @@ class StatisticsOpensRepository extends Repository {
       ->getQuery()
       ->getSingleScalarResult();
     if ($newslettersSentCount < 3) {
+      $subscriber->setEngagementScore(null);
       $this->entityManager->flush();
       return;
     }