Implement simplified commercials workflow

It's now possible to simply enter the url of the third party provider to enter a commercial.

db/migrate/20150417050953_add_url_to_commercial.rb

@@ -0,0 +1,11 @@
+class AddUrlToCommercial < ActiveRecord::Migration
+  class TempCommercial< ActiveRecord::Base
+    self.table_name = 'commercials'
+  end
+
+  def change
+    add_column :commercials, :url, :string
+
+    # Don't delete commercial_type and commercial_id for backward compatibility
+  end
+end