Add entity trait for autoincremented IDs

[MAILPOET-2014]

lib/Doctrine/EntityTraits/AutoincrementedIdTrait.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace MailPoet\Doctrine\EntityTraits;
+
+trait AutoincrementedIdTrait {
+  /**
+   * @Column(type="integer")
+   * @Id
+   * @GeneratedValue
+   * @var int|null
+   */
+  private $id;
+
+  /** @return int */
+  public function getId() {
+    return $this->id;
+  }
+}