erp5_xhtml_style: use Base_redirect on logged_out page.

Otherwise, the portal_status_message is not correctly escaped, which can cause bugs with older web browsers. It happens when the translation of this messages in the user's language uses special characters

product/ERP5/bootstrap/erp5_xhtml_style/SkinTemplateItem/portal_skins/erp5_xhtml_style/logged_out.zpt

 <tal:block xmlns:tal="http://xml.zope.org/namespaces/tal">
-  <tal:block tal:define="dummy python:request.RESPONSE.redirect('%s?portal_status_message=%s' % (here.absolute_url(), here.Base_translateString('You have been logged out.')))" />
+  <tal:block tal:define="dummy python: context.Base_redirect(here.absolute_url(), {'portal_status_message':here.Base_translateString('You have been logged out.')})" />
 </tal:block>
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