Optimize memory usage for batch processing of template images

NewsletterTemplateEntity carries base64 data of image. This commit changes how we load templates entities when we process thumbnail images so that there is only one template in memory.
[MAILPOET-2686]

lib/Doctrine/Repository.php

@@ -103,6 +103,13 @@ abstract class Repository {
     $this->entityManager->flush();
   }
 
+  /**
+   * @param T $entity
+   */
+  public function detach($entity) {
+    $this->entityManager->detach($entity);
+  }
+
   /**
    * @return class-string<T>
    */