erp5_web_renderjs_ui: the web_section is not always set in request, try to look at parent

Make sure we find section if we render https://my_site/my_section/foo.html. This is required
to take into account configuration of the web section, like specific content security policy.

bt5/erp5_web_renderjs_ui/SkinTemplateItem/portal_skins/erp5_web_renderjs_ui/WebPage_viewAsWeb.py

@@ -5,6 +5,10 @@ if response is None:
 
 web_page = context
 web_section = REQUEST.get("current_web_section")
+if web_section is None:
+  parent_value = context.getParentValue()
+  if parent_value.getPortalType() == "Web Section":
+    web_section = parent_value
 
 if REQUEST.getHeader('If-Modified-Since', '') == web_page.getModificationDate().rfc822():
   response.setStatus(304)