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
fix bug 1051877 - Launch signin windows/links when icons within header login when clicked
Fix bug 1055708 - Refactor AllAuth front-end JavaScript and prevent multiple JS inclusions
Add test for ensuring Persona form is provided to the page
Update signup page text via Stephanie's comments
Adding code comments to Persona login
Fix login-on-mobile AJAX request race condition