Pull some Structuring and Testing Data content back to Onboarding

Onboarding Sprint 3 is pretty light on content in both the Prep and Day Plan.

Also, completing the Onboarding module is currently our sign we can enrol people because they're likely to succeed. But Onboarding doesn't actually teach much new technical content - it has people apply their ITD knowledge (HTML+CSS) on GitHub (which is new).

Ideally we could pull some JS back into Onboarding, both to give some meaningful content to the sprint, and to make sure people have learnt some JS before enrolling.