Display only poll stats for used channels

So if there's no vote using by mail (which is the case in some places),
no stats related to voting via mail are displayed.

app/models/poll/stats.rb

@@ -21,6 +21,10 @@ class Poll::Stats
     total_participants_web + total_participants_booth
   end
 
+  def channels
+    CHANNELS.select { |channel| send(:"total_participants_#{channel}") > 0 }
+  end
+
   CHANNELS.each do |channel|
     define_method :"total_participants_#{channel}" do
       send(:"total_#{channel}_valid") +
@@ -58,7 +62,7 @@ class Poll::Stats
   end
 
   def total_letter_valid
-    0 # TODO
+    voters.where(origin: "letter").count # TODO: count only valid votes
   end
 
   def total_letter_white