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
also the reason this isn't showing up right, is that the emit needs to now be moved into where it completes the QueueItem.update at this section of code. The data object it emits should also contain the new updated QueueItem data (with the thumbnail basically)