Add A/B test for responsive grid.

[beausmith]

extra-large

large

medium

mobile

[beausmith]

@roblafave Take a look at this PR: https://d2-staging-pr-219.herokuapp.com/ (you may have to open incognito to get the "responsive" a/b test)

[cee-dub]

Took me a few rounds of clear cookies and reload, using the dev console, but got it! I also made sure A/B tests are active all the time in non-production environments so we can look at them in review apps and staging.