serializer on accompanying course

Two new routes:

* `GET /{_locale}/person/api/1.0/accompanying-course/{parcours_id}/show.json`: get a json representation for a course
* `POST /{_locale}/person/api/1.0/accompanying-course/{parcours_id}/participation.json`:
add a particitipation to course. Usage:

    `curl -v --cookie "PHPSESSID=fed98aa23e40cb36e630f84155aea3bb;" -X
POST --data '{ "id": 481 }'
http://localhost:8001/fr/person/api/1.0/accompanying-course/270/participation.json`

    Will add the person with id "481" to the course.

src/Bundle/ChillPersonBundle/config/services/controller.yaml

@@ -40,4 +40,8 @@ services:
         tags: ['controller.service_arguments']
 
     Chill\PersonBundle\Controller\AccompanyingCourseController:
+        arguments:
+            $serializer: '@Symfony\Component\Serializer\SerializerInterface'
+            $dispatcher: '@Symfony\Contracts\EventDispatcher\EventDispatcherInterface'
+            $validator: '@Symfony\Component\Validator\Validator\ValidatorInterface'
         tags: ['controller.service_arguments']