FIX: Award 'First Onebox' badge just for Oneboxed URLs. (PR #7974)

We are adding a onebox class when cooking the content to any link, but we are not sure it will be oneboxed until we actually try to onebox it. This commit ensures that.


You’ve signed the CLA, nbianca. Thank you! This pull request is ready for review.

This pull request has been mentioned on Discourse Meta. There might be relevant details there: