A first VueJS component, get api datas, a simple button add person

* !! composer require serializer !!
* see: http://localhost:8001/fr/parcours/861/show

src/Bundle/ChillPersonBundle/Entity/Person.php

@@ -369,7 +369,7 @@ class Person implements HasCenterInterface
     /**
      * Return the opened accompanying period.
      */
-    public function getOpenedAccompanyingPeriod() : AccompanyingPeriod
+    public function getOpenedAccompanyingPeriod() : ?AccompanyingPeriod
     {
         if ($this->isOpen() === false) {
             return null;