Use singular in entity names

[MAILPOET-2439]

lib/Entities/StatisticsOpenEntity.php

@@ -0,0 +1,39 @@
+<?php
+
+namespace MailPoet\Entities;
+
+use MailPoet\Doctrine\EntityTraits\AutoincrementedIdTrait;
+use MailPoet\Doctrine\EntityTraits\CreatedAtTrait;
+use MailPoet\Doctrine\EntityTraits\UpdatedAtTrait;
+use MailPoetVendor\Doctrine\ORM\Mapping as ORM;
+
+/**
+ * @ORM\Entity()
+ * @ORM\Table(name="statistics_opens")
+ */
+class StatisticsOpenEntity {
+  use AutoincrementedIdTrait;
+  use CreatedAtTrait;
+  use UpdatedAtTrait;
+
+  /**
+   * @ORM\OneToOne(targetEntity="MailPoet\Entities\NewsletterEntity")
+   * @ORM\JoinColumn(name="newsletter_id", referencedColumnName="id")
+   * @var NewsletterEntity|null
+   */
+  private $newsletter;
+
+  /**
+   * @ORM\OneToOne(targetEntity="MailPoet\Entities\SendingQueueEntity")
+   * @ORM\JoinColumn(name="queue_id", referencedColumnName="id")
+   * @var SendingQueueEntity|null
+   */
+  private $queue;
+
+  /**
+   * @ORM\Column(type="integer")
+   * @var int|null
+   */
+  private $subscriber_id;
+
+}