Add cron job for generating/re-generating template thumbnail files

[MAILPOET-2686]

tests/integration/Cron/DaemonHttpRunnerTest.php

@@ -306,6 +306,8 @@ class DaemonHttpRunnerTest extends \MailPoetTest {
       'createSubscribersEngagementScoreWorker' => $worker,
       'createSubscribersLastEngagementWorker' => $worker,
       'createSubscribersCountCacheRecalculationWorker' => $worker,
+      'createReEngagementEmailsSchedulerWorker' => $worker,
+      'createNewsletterTemplateThumbnailsWorker' => $worker,
     ]);
   }
 }