fix plugin permissions for child plugins on apphook pages

If discovered a bug with child plugins on apphook pages with placeholder field. The problem is in the render_plugin tag. There are permissions checked for the plugin against the public version of the page because on apphook pages / subpages there is always the public version present in `current_page`. The page permissions on the other hand are only stored for the draft version so no one except super users have permissions on the live version of a page. The result is that there are no edit menus displayed for child plugins in the placeholders (structure mode) making it impossible to edit them.

I implemented a quick fix for this but it should be discussed if the fix should be implemented deeper down in the stack at page object level.