Thingiverse Thumbnails

Fixes https://github.com/skalnik/print-queue/issues/33

Fetches the Preview URL of Thingiverse Things and saves it in redis so we can use it for admin viewing
I'm not sure if the update is running properly in my local build. I'm seeing each queue item in the jobData obj come back with no thumbnail entry in it. This results in a broken admin page, because it cannot bind the thumbnail when it can't find the thumbnail key when it iterates.