Changed DB structure the way, many more elements can have attachments

Also every Element has now a preview attachment field. For the user this will be used as profile picture.

src/Entity/Attachments/PartAttachment.php

@@ -35,7 +35,7 @@ use App\Entity\Parts\Part;
 use Doctrine\ORM\Mapping as ORM;
 
 /**
- * Class PartAttachment
+ * A attachment attached to a part element.
  * @package App\Entity
  * @ORM\Entity()
  */
@@ -43,7 +43,7 @@ class PartAttachment extends Attachment
 {
 
     /**
-     *
+     * @var Part The element this attachment is associated with.
      * @ORM\ManyToOne(targetEntity="App\Entity\Parts\Part", inversedBy="attachments")
      * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
      */
@@ -56,7 +56,6 @@ class PartAttachment extends Attachment
         }
 
         $this->element = $element;
-
         return $this;
     }
 }