Updated `serializers.py`.

1. Updating import order and using explicit relative import.

2. Added the field `product`, then the client app can POST a new review for specific product.
Refactoring `views.py`

1. Reordered the import orders.
2. Replace the class `ReviewDetail` with `ReviewViewSet`.
3. Added the rendering content type `text/javascript` to the view `ng_products_js`.
Added a newline in end of file.
Removed the field `modified_on` in the model `Review`.
Updated git ignoring.
Moved `app.js` from templates dir to static dir.

Since it doesn't have dynamic contents.
Refactor the review form controller.

Now it can POST the user reviews via RESTful API.

Notice that the csrf token setting is necessary when you have the session (ex., when you are authenticated or log in).