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](https://github.com/mozilla/kuma/blob/master/apps/wiki/views.py#L324) to check in the en-US locale first and find the fr alternative for an existing en-US doc before throwing a 404 error.

* https://developer.mozilla.org/docs/Mozilla/Firefox_OS/Security -> https://developer.mozilla.org/fr/docs/Mozilla/Firefox_OS/securite