[bug 846756] Add A/B test for product ordering on /products

?b returns the first two products in reverse order.

This commit adds the custom script for the google content experiment (see f3ca0444b802b53bc90134c46eb0fcc3e53de75c for the last one we did successfully). The I tweak the view so that passing `?b` reverses the order of the first two products.

That's all folks.

r?