[FIX] web: avoid incoherent actual_mode after refresh

When accessing an existing record in form mode directly (enter the url or refresh a page), the daterecord has not been initialized yet. This means that the value of actual_mode will be set to 'edit' before loading the current record (method _actualize_mode() called from do_show()).
This was problematic for one2many fields that we loaded in edit mode, showing add/delete icons/buttons in readonly views. (opw 607910)