auto screenshot with gist-snap

use or borrow from gist-snap to generate images on the server on behalf of the user
https://github.com/1wheel/gist-snap

since we already clone gists on demand for a user when we upload the thumbnail we could kick this off as a different backend process.

The issue names gist-snap and the existing gist-cloning and thumbnail-upload flow; read gist-snap first, then trace that upload path. Done means the server can generate a screenshot for a user's cloned gist through a separate backend process and use the resulting image as the thumbnail.

Written by the indexing model from the issue text.