fix bug 887428: Redirect for misprinted URL in promo materials

This seems to work. I tried it with a few different locales set in my browser, and they went to the right spots.

It even captures the case where the fr URL has changed from Security to securite, because the document view handler does some gymnastics to check in the en-US locale first and find the fr alternative for an existing en-US doc.

* https://developer.mozilla.org/docs/Mozilla/Firefox_OS/Security -> https://developer.mozilla.org/fr/docs/Mozilla/Firefox_OS/securite