Event: move validation to annotation and add UniqueEntity constraint on Participation

src/Bundle/ChillEventBundle/Controller/ParticipationController.php

@@ -236,6 +236,7 @@ final class ParticipationController extends AbstractController
         return $this->render('@ChillEvent/Participation/new.html.twig', [
             'form' => $form->createView(),
             'participation' => $participation,
+            'ignored_participations' => [],
         ]);
     }