Add actions to render plain participant lists

Provide URLs
http://www.math.univ-toulouse.fr/top-geom-conf-2013/data/participants/by_conference/Intro
http://www.math.univ-toulouse.fr/top-geom-conf-2013/data/participants/by_conference/GESTA
http://www.math.univ-toulouse.fr/top-geom-conf-2013/data/participants/by_conference/Llagone
http://www.math.univ-toulouse.fr/top-geom-conf-2013/data/participants/by_conference/CoMB
to get plain lists of participants for each conference.