Only show recommended tab for opt-in users (bug 1097332)

I see you're trying to be generic with the `.enabled-if` class name, but you're already [special-casing the recommendations in the JS](#diff-5135d1767348467a1d4abe7d4abf7b3eR36) in a non-generic way.

so can you remove the `special_class` stuff and just target `.show-recommendations .tab-item.recommendations`?