When we request multiple participations on an event, and all people already
participate on the event, the controller redirected to a route which provoked
a bad request error. Now, we redirect to the view route.
The participationController accept a new parameter : `persons_ids`,
which must receive a comma-separated list of person ids. A participation
will be create for all those peoples.
The `new` and `create` actions does not allow to receive both
`person_id` and `persons_ids`.
Tests are added.
ref #6
