Create REST endpoints to upload image of a product.

- implement post/get/patch/delete requests to upload/retrieve image of a product.
- create a model and serializer for product photos.
- uploading photo using POST method shows a HTML form, if no image is send in request.data (saved in templates folder).
- Changed some HTTP status messages for consistency.
	- Use HTTP_201_NO_CONTENT when item is deleted.
  - Use HTTP_404_NOT_FOUND when item is not found in database.