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/ChillMainBundle/Serializer/Normalizer/DateNormalizer.php

@@ -0,0 +1,39 @@
+<?php
+/*
+ *
+ * Copyright (C) 2014-2021, Champs Libres Cooperative SCRLFS, <http://www.champs-libres.coop>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ *  License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+namespace Chill\MainBundle\Serializer\Normalizer;
+
+use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
+
+class DateNormalizer implements NormalizerInterface
+{
+    public function normalize($date, string $format = null, array $context = array())
+    {
+        /** @var \DateTimeInterface $date */
+        return [
+            'datetime' => $date->format(\DateTimeInterface::ISO8601),
+            'u' => $date->getTimestamp()
+        ];
+    }
+
+    public function supportsNormalization($data, string $format = null): bool
+    {
+        return $data instanceof \DateTimeInterface;
+    }
+}