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/config.json

@@ -1,7 +1,6 @@
 {
   "entry": {
     "main": [
-      "./scripts/util/public-path.js",
       "./scripts/main.js",
       "./styles/main.scss"
     ],
@@ -10,5 +9,7 @@
     ]
   },
   "publicPath": "/app/themes/sage",
-  "devUrl": "http://example.dev"
+  "devUrl": "https://example.dev",
+  "proxyUrl": "https://localhost:3000",
+  "cacheBusting": "[name]_[hash:8]"
 }