Fix Bug 1034859 - [404] https://www.mozilla.org/en-US/about/buttons/MoCo_160x280.png

I'm not sure if it matters, but these buttons only exist in the /en-US/about/buttons/ dir. I suppose it's possible that there are links/requests with other (or no) locale code in the URL.

I have also asked [a question about the necessity of these images over on the bug](https://bugzilla.mozilla.org/show_bug.cgi?id=1034859#c6).