Add mail votes stub to poll stats

So far we don't know about implemenation details.

app/models/poll/stats.rb

@@ -2,22 +2,26 @@ class Poll::Stats
   include Statisticable
   alias_method :poll, :resource
 
+  CHANNELS = %i[web booth mail]
+
   def self.stats_methods
     super +
       %i[total_valid_votes total_white_votes total_null_votes
          total_participants_web total_web_valid total_web_white total_web_null
          total_participants_booth total_booth_valid total_booth_white total_booth_null
+         total_participants_mail total_mail_valid total_mail_white total_mail_null
          total_participants_web_percentage total_participants_booth_percentage
-         valid_percentage_web valid_percentage_booth total_valid_percentage
-         white_percentage_web white_percentage_booth total_white_percentage
-         null_percentage_web null_percentage_booth total_null_percentage]
+         total_participants_mail_percentage
+         valid_percentage_web valid_percentage_booth valid_percentage_mail total_valid_percentage
+         white_percentage_web white_percentage_booth white_percentage_mail total_white_percentage
+         null_percentage_web null_percentage_booth null_percentage_mail total_null_percentage]
   end
 
   def total_participants
     total_participants_web + total_participants_booth
   end
 
-  %i[web booth].each do |channel|
+  CHANNELS.each do |channel|
     define_method :"total_participants_#{channel}" do
       send(:"total_#{channel}_valid") +
         send(:"total_#{channel}_white") +
@@ -53,8 +57,20 @@ class Poll::Stats
     recounts.sum(:null_amount)
   end
 
+  def total_mail_valid
+    0 # TODO
+  end
+
+  def total_mail_white
+    0 # TODO
+  end
+
+  def total_mail_null
+    0 # TODO
+  end
+
   %i[valid white null].each do |type|
-    %i[web booth].each do |channel|
+    CHANNELS.each do |channel|
       define_method :"#{type}_percentage_#{channel}" do
         calculate_percentage(send(:"total_#{channel}_#{type}"), send(:"total_#{type}_votes"))
       end