[wip] Run the node.js backend in Vagrant

Currently, the node.js sponsorship-form backend isn't actually launched in our Vagrant dev environment. This will run it right from the rsync'd `/vagrant_data/src/sponsorship-form` directory.

/cc @kenperkins, @ycombinator 
It's kind of weird to me to mess with the site content in a role called "backend". I didn't know what else to call it, though. Any ideas?