Tidy-up and document templates

Some commenting and indenting added.
Correct URL regex for reviews

Had fallen out of sync with the core product URLs as it still used a
hyphen instead of an underscore.
Rework review templates

* Extract out the review snippet into its own partial.
* Bring the review detail inline with the review list template.
Rename review templates

Bring names inline with normal django conventions.
Remove superfluous view

Plus some light tidying of the views module
Move management command to conventional location
Move update_product_rating onto Product model

It seems to belong conceptually there, rather than on the ProductReview
model.