Don't load @variants on index

They aren't used on index, they're only used in the form.
Pull out eager loads on index from others

We only access images through the variant_images association, which
isn't covered through the variants_including_master

Variants_including_master will speed up the form, and this will speed up
the index.
Cache the result of has_products?

Rather than calling it twice... which will hit the database
Merge pull request #2 from freerunningtech/product_admin_speedups

Product admin speedups