contents.js wrapper for contents service

This is PR #6671, rebased because it's a blocker for the text editor components, and @fperez asked me to make that high priority. So far, I've just rebased, I intend to test it and fix things I see.
Should be `new(type, path, options)`, so that `new_notebook` becomes `new('notebook', path, options)`, populating the `type` field in the request model.