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.

app/assets/stylesheets/stats.scss

@@ -40,6 +40,18 @@
       content: image-url('stats_woman.png');
     }
 
+    &.web::before {
+      content: image-url('stats_web.png');
+    }
+
+    &.booth::before {
+      content: image-url('stats_booth.png');
+    }
+
+    &.mail::before {
+      content: image-url('stats_mail.png');
+    }
+
     .content {
       display: inline-block;
       margin-right: $line-height * 2;