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

As for bug 1038541, the pages have been moved to webplatform.

Now also using proper naming for the Web Platform site on the
'Learn HTML' page.
Merge pull request #2696 from vrubiolo/bug-1056487-rework

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