Abstract the Procedure whitelisting

app/models/procedure.rb

@@ -147,6 +147,10 @@ class Procedure < ActiveRecord::Base
     publiee? || archivee?
   end
 
+  def whitelisted?
+    whitelisted_at.present?
+  end
+
   def total_dossier
     self.dossiers.state_not_brouillon.size
   end

app/views/manager/procedures/show.html.haml

@@ -20,7 +20,7 @@
 %header.header
   %h1.header__heading= content_for(:title)
   .header__actions
-    - if procedure.whitelisted_at.nil?
+    - if !procedure.whitelisted?
       = link_to 'whitelister', manager_procedure_whitelist_path(procedure), method: :post, class: 'button'
 
 %dl