Fix bug 1056487 - broken Opera Dev links on MDN 'Learn Javascript' page

As for bug 1038541, the pages have been moved to webplatform.
Thanks for pointing this out. I have updated the 'Learn HTML' page as well (cf my other bug report). I was not sure how to proceed w.r.t the pull request (force push of the rewritten commit?) so I have put the changes into another, rebased branch at https://github.com/mozilla/kuma/pull/2696