Bugfix in `preview_page`

Replaced call to `cms.utils.plugins.current_site` with one to the regular `django.contrib.sites.models.get_current_site`.

`current_site` looks at as session variable that indicates the site you're editing in the admin, but in this view we want the actual Site of the Django project we're running in so that we know whether the domaminname should be in the redirect.