fix front page issue

actually the front page is broken when Access Tab Front Page is enabled

It's because erp5_wendelin_configurator is now in the default_bt5, and it has ERP5Site_createDefaultKnowledgeBox which refer to portal_gadgets/WendelinInformationGadget present at erp5_wendelin

so if erp5_wendelin is not installed, we'll have such error

i propose to move ERP5Site_createDefaultKnowledgeBox to erp5_wendelin

@Tyagov @jerome @tsoulard what do you think ?