Add tasks to generate and expire stats cache

We're generating stats every 2 hours because it's less than the time it
will take to generate stats for every process. Once stats are generated,
this task should take less than a second.

The regenerate task has been added so we can manually execute it.

config/schedule.rb

@@ -27,6 +27,10 @@ every 1.day, at: "5:00 am" do
   rake "-s sitemap:refresh"
 end
 
+every 2.hours do
+  rake "-s stats:generate"
+end
+
 # Temporally not send dashboard's notifications
 # every 1.day, at: "7:00 am" do
 #   rake "dashboards:send_notifications"

lib/tasks/stats.rake

@@ -0,0 +1,24 @@
+namespace :stats do
+  desc "Generates stats which are not cached yet"
+  task generate: :environment do
+    Budget.find_each do |budget|
+      Budget::Stats.new(budget).generate
+      print "."
+    end
+
+    Poll.find_each do |poll|
+      Poll::Stats.new(poll).generate
+      print "."
+    end
+  end
+
+  desc "Expires stats cache"
+  task expire_cache: :environment do
+    [Budget, Poll].each do |model_class|
+      model_class.find_each { |record| record.find_or_create_stats_version.touch }
+    end
+  end
+
+  desc "Deletes stats cache and generates it again"
+  task regenerate: [:expire_cache, :generate]
+end