[web] set pageid at request instanciation rather than in htmlheader template In some occasions, we need to force a pageid: typically an ajax-rendererd creation form would need to use the pageid of the main page to correclty set it's page_data (e.g. rql varmker). If pageid is found in req.form, it's used, otherwise we build a new one with. make_uid(). The behaviour should be consistent with the former one in JSonController.