Use 64x64 icons (resized to 32x32) in transonic (bug 1088369)

https://bugzilla.mozilla.org/show_bug.cgi?id=1088369

The goal is to remove 32x32 icons from all Feed APIs to make the APIs Fireplace uses even smaller. I could do that with a custom serializer but thought it'd be easier to just switch transonic (the only consumer of 32x32 icons) to 64x64. Let me know what you guys think of that approach, that makes the zamboni part trivial and simple to maintain, and doesn't require changes to fireplace.