make os_files_private an OS App; verify tests locally #1028
incorporate review by @rbran100 #1165

Move from case "init" to case "node", remove superfluous object
condition.

Verify node is "Group Content" first, then verify node is in the
current group (stored in $this->id) and if not, redirect away.

Then, check $this->purl_provider == "vsite_domain" and finally redirect
when purl is present.