Add poll stats by channel

Note we currently don't have a way to get the votes by mail, so that
section is always blank.

config/locales/en/stats.yml

@@ -15,4 +15,8 @@ en:
     geozone: "District"
     geozone_participation: "% District population participation"
     polls:
+      by_channel: "Participants by channel"
       vote_by_channel: "Vote type by channel"
+      web_percentage: "%{percentage} Web"
+      booth_percentage: "%{percentage} Booths"
+      mail_percentage: "%{percentage} Mail"

config/locales/es/stats.yml

@@ -15,4 +15,8 @@ es:
     geozone: "Distrito"
     geozone_participation: "% Partipación población del distrito"
     polls:
+      by_channel: "Participación por medio"
       vote_by_channel: "Votos emitidos por medio"
+      web_percentage: "%{percentage} Web"
+      booth_percentage: "%{percentage} Urnas"
+      mail_percentage: "%{percentage} Correo"