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/util/mergeWithConcat.js

@@ -0,0 +1,14 @@
+/* eslint-disable import/no-extraneous-dependencies */
+
+const mergeWith = require('lodash/mergeWith');
+
+module.exports = function mergeWithConcat() {
+  const args = [].slice.call(arguments);
+  args.push((a, b) => {
+    if (Array.isArray(a) && Array.isArray(b)) {
+      return a.concat(b);
+    }
+    return undefined;
+  });
+  return mergeWith.apply(this, args);
+};