Make sure only assigned reviewers can review a paper
Make sure only assigned reviewers can review a paper

Currently, to verify that a user can review a paper
(in peer reviewing), we only check if the user is part of
the reviewing team, instead of checking if they've been
assigned to the specific paper.

This change fixes that by ensuring they've been assigned to the paper.

indico/indico,indico/indico,indico/indico,indico/indico