Only generate stats if we can access them

There's no point generating stats nobody can access.

Note with this change we're automatically excluding polls created in the
dashboard, since these polls don't have stats enabled.

lib/tasks/stats.rake

@@ -2,13 +2,14 @@ namespace :stats do
   desc "Generates stats which are not cached yet"
   task generate: :environment do
     ApplicationLogger.new.info "Updating budget and poll stats"
+    admin_ability = Ability.new(Administrator.first.user)
 
-    Budget.find_each do |budget|
+    Budget.accessible_by(admin_ability, :read_stats).find_each do |budget|
       Budget::Stats.new(budget).generate
       print "."
     end
 
-    Poll.find_each do |poll|
+    Poll.accessible_by(admin_ability, :stats).find_each do |poll|
       Poll::Stats.new(poll).generate
       print "."
     end