Turn on nginx gzip compression for javascript & css assets

Enable nginx [gzip compression](https://developers.google.com/speed/docs/insights/EnableCompression) for core `js` and `css` assets.

Statistics for the change in a moderately sized app:

| File  | Before | After | Reduction |
| ------------- | ------------- | ------------- | ------------- |
| `vendor.css` | `71k` | `48.2k` | 32%
| `app.css`  | `184k` | `38.4k` | 79%
| `vendor.js`  | `663k` | `223k` | 66%
| `app.js`  | `260k` | `51.3k` | 80%