Write restful service to find all orphans

To help fix recent bugs re orphaned discussions and
to prototype a more restful json oriented api.

@chrisndodge @singingwolfboy Please review

It could use a real gui at some point, but for now can invoke by logging into studio and then changing url to something like host/org.course/orphan (e.g., http://localhost:8001/test.sep3/orphan) which dumps a json doc into browser w/ the ids for the orphans.