Implement context-specific voters for all current entities that can be linked to a document

For reusability an AbstractStoredObjectVoter was created and a StoredObjectVoterInterface.
A WorkflowDocumentService checks whether the StoredObject is involved in a workflow.

src/Bundle/ChillDocStoreBundle/Repository/AssociatedEntityToStoredObjectInterface.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace Chill\DocStoreBundle\Repository;
+
+use Chill\DocStoreBundle\Entity\StoredObject;
+
+interface AssociatedEntityToStoredObjectInterface
+{
+    public function findAssociatedEntityToStoredObject(StoredObject $storedObject): ?object;
+}