Likes

This branch contains implementation of 'likes' functionality for vendor products.

- 'like' button is visible on product cards.
- button also shows total likes count.
- 'like' button is disabled for anonymous user.
- authenticated user can like a product. If product is already liked, user can unlike the same product.
- 'like' button is responsive and shows updated total likes count.

Front-end : Implemented UI, controller, service.
Back-end : added model, implemented views, REST endpoints. Also updated unit test cases for products.

Make sure to install 'django-secretballot'.  [ pip install django-secretballot ]