Add `Spree::Promotion#products` back.

This was removed in 6ff0e82, however it's still currently used in [frontend/app/views/spree/products/_promotions.html.erb](https://github.com/spree/spree/blob/master/frontend/app/views/spree/products/_promotions.html.erb#L9-L15)

So I added it back. Alternatively, we could remove it from the front end.