Feature/upload with preview

Shows "upload" button on the preview screen. You can upload your gadget right from there. Currently it fails with an error if version you are trying to upload is already in your sandbox. Maybe we can do better at some point. One of the ideas - to append number to version every time you preview: 1.2.3-120 (by @possibilities)

Also it is quite ugly. I am planning on solving that by bringing design-theme in here at some point.