Webpacker is the ruby gem that wraps webpack, a javascript utility
for transforming and bundling.

Add:
- Yarn, which webpacker uses to manage javascript dependencies.
- .babelrc, configuration file for Babel, the standard javascript
transpiler. We're using it to transpile es6 javascript so that it's
compatible with more browsers.
- Config files for webpacker.
- Config files to allow webpacker to process .erb files.

- Get the latest node.js from NodeSource, officially recommended
linux binaries.
link: https://github.com/nodesource/distributions
- Install yarn, javascript package manager required by webpacker.
- Create separate volume for /refugerestrooms/node_modules in
Docker to prevent mounting /refugerestrooms from overriding what
was installed by yarn.

All changes cherry-picked and squashed from decaffeinate branch

Remove jquery-rails gem because it's now managed with yarn

Now we can write es6 code in any .js file! All the coffeescript
has been converted into js.

Changes:
- Use es6 modules instead of globals. This is better encapsulation.
This results in the removal of `window.Refuge`. Instead, to use a
module, like Geocoder, just import it like this:
`import { Geocoder } from 'path/to/geocoder';`

Updates the 'webpacker' branch to use Ruby 2.3.7