erp5_knowledge_pad: validate pad in its context

For example, this is required for multi-valued fields when an unknown value
is kept selected. Like elsewhere in ERP5, we don't want validation to raise
in this case.

bt5/erp5_knowledge_pad/SkinTemplateItem/portal_skins/erp5_knowledge_pad/KnowledgeBox_baseEdit.xml

@@ -64,7 +64,7 @@ box = context.restrictedTraverse(box_relative_url)\n
 portal_selection = context.getPortalObject().portal_selections\n
 \n
 # do validation\n
-form = getattr(context,form_id)\n
+form = getattr(box, form_id)\n
 try:\n
   # Validate\n
   form.validate_all_to_request(request, key_prefix=form_fields_main_prefix)\n