Refactor build routine and switch to airbnb style (#1703)

- Relocate webpack stuff to assets/build
- Switch all js to airbnb code style
- Update webpack to v2.0 (beta)
- Update all other dependencies
- Assets manifest uses relative path as key [1]
- Code should be compatible with Node 4.5.0
- All dependencies have been updated

[1]: Previously the assets manifest would use the basename of a file path as
its key when looking up a cachebusted file. This was to be consistent with
Sage 8. This change makes it so that the relative path is used instead.

To clarify, review the following example.

Before:
```
{
  "main.js": "scripts/main_5f4bfc9a9f82291c6dea.js",
  "main.css": "styles/main_5f4bfc9a9f82291c6dea.css",
  "customizer.js": "scripts/customizer_5f4bfc9a9f82291c6dea.js"
}
```

After:
```
{
  "scripts/main.js":"scripts/main_5f4bfc9a9f82291c6dea.js",
  "styles/main.css":"styles/main_5f4bfc9a9f82291c6dea.css",
  "scripts/customizer.js":"scripts/customizer_5f4bfc9a9f82291c6dea.js"
}
```

assets/build/webpack.config.watch.js

@@ -0,0 +1,22 @@
+/* eslint-disable import/no-extraneous-dependencies */
+const BrowserSyncPlugin = require('browser-sync-webpack-plugin');
+const url = require('url');
+
+const config = require('./config');
+
+module.exports = {
+  output: { pathinfo: true },
+  debug: true,
+  devTool: 'cheap-module-source-map',
+  plugins: [
+    new BrowserSyncPlugin({
+      host: url.parse(config.proxyUrl).hostname,
+      port: url.parse(config.proxyUrl).port,
+      proxy: config.devUrl,
+      files: [
+        'templates/**/*.php',
+        'src/**/*.php',
+      ],
+    }),
+  ],
+};