Add new `render_product` templatetag

This looks for an overriding template for the product partial, based on
the product's UPC and product class.  This hook allows projects to
provide custom partials for changing how a product appears in browsing
views.
render_product: Docs & default paths
Use new render_product tag for product carousels
Merge pull request #601 from tangentlabs/issue/601/alternative-partials

Support alternative product partials