Add SHOP_USE_RATINGS (default True), allowing ratings to be disabled.

Motivated by a small store, for which ratings are irrelevant (you're either buying the sole product or you aren't :)

The conditional in ``category.html`` might be better done as a context processor-supplied filtered view of ``SHOP_PRODUCT_SORT_OPTIONS``, but I didn't see one to piggyback on.  Since this the only template that uses it, my first approach is the lazy one; let me know if you prefer something else..