Added some simple time travel mechanism for part view.

In the moment it is not possible to show elements that were deleted.

src/Entity/Base/AbstractDBElement.php

@@ -56,7 +56,7 @@ use Symfony\Component\Serializer\Annotation\Groups;
  *      "user" = "App\Entity\User"
  *  })
  */
-abstract class AbstractDBElement
+abstract class AbstractDBElement implements \JsonSerializable
 {
     /** @var int|null The Identification number for this part. This value is unique for the element in this table.
      * Null if the element is not saved to DB yet.
@@ -93,4 +93,9 @@ abstract class AbstractDBElement
      * @return string The ID as a string;
      */
     abstract public function getIDString(): string;
+
+    public function jsonSerialize()
+    {
+        return ['@id' => $this->getID()];
+    }
 }