Introduced the shortcut `render()` to render the template.
Introduced the template to view `product_detail`.
Introduced the shortcut `get_object_or_404()`.

It's a very common idiom to use `get()` and raise `Http404` if the object doesn't exist, therefore used a shortcut `get_object_of_404()` instead.
Ignored dir `frontend_demo` and removed caches of it in Git.