Update the poll plugin to new security domain setup

The '/polldata' security domain can be setup as an hierarchical domain now and instead of assigning readwrite / read to the site user groups, assign them to single site users since the HST groups for liveusers, previewusers and sitewriters have been removed

One small thing then still is a problem: Some code is executed where a folder node (the root content folder for a site) is trying to be read by a sitewriter. By default, a sitewriter does not have read access any more on folders. Hence, replace that code logic such that you do not need read access to the folders for sitewriter