Use Doctrine references when saving links

This commit changes the code of the method
\MailPoet\Newsletter\Links\Links::save() to use Doctrine references
instead of full entities when getting a newsletter and sending queue
entities to save the related links.

We don't need the full entities here, and getting a full entity for the
sending queue was generating an error in the
testItLogsErrorWhenNewlyRenderedNewsletterBodyIsInvalid() integration
test (see the description of PR
https://github.com/mailpoet/mailpoet/pull/3784 for more details).

To be able to get the reference, it was necessary to add a helper method
called getReference() to \MailPoet\Doctrine\Repository.

[MAILPOET-3816]

lib/Doctrine/Repository.php

@@ -103,6 +103,10 @@ abstract class Repository {
     $this->entityManager->flush();
   }
 
+  public function getReference($id) {
+    return $this->entityManager->getReference($this->getEntityClassName(), $id);
+  }
+
   /**
    * @param T $entity
    */