Setup NPM with an empty package.json file

Note that, even if we're excluding the `node_modules/` folder from
version control, we aren't adding it to Capistrano's shared folders
because, when `node_modules` is a symbolic link, NPM removes it when
running `npm install`.

.gitignore

@@ -26,6 +26,7 @@ tmp/
 /public/assets/
 /public/machine_learning/data/
 /public/tenants/*/machine_learning/data/
+/node_modules/
 
 # Bundler config, cache and gemsets
 .bundle/