Accept an optional format in rocketfuel image URLs (bug 944109)

https://bugzilla.mozilla.org/show_bug.cgi?id=944109

This should fix the collection image URLs be totally backwards-compatible with what we had before (rocketfuel was PUTing to `/api/v1/rocketfuel/collections/xxx/image/`) and at the same time give us nice URLs with a `.png` suffix to make the CDN happy.