Do not block transition in EntityWorkflow when the user is member of a dest user group

- refactor EntityWorkflowGuardTransition + tests
- allow to find easily user within userGroup by adding a dedicated method to UserGroup::contains

src/Bundle/ChillMainBundle/Entity/UserGroup.php

@@ -141,4 +141,9 @@ class UserGroup
     {
         return true;
     }
+
+    public function contains(User $user): bool
+    {
+        return $this->users->contains($user);
+    }
 }