redirect to STL from thingiverse thing URL

In the vein of https://github.com/github/make_me/pull/7, this will redirect to the STL from a regular Thingiverse 'thing' URL.

It's not as easy or simple as the Tinkercad stuff because the IDs don't match (because a Thing can actually be multiple STLs). The page has to be downloaded and parsed, then the first STL link is taken.

:cyclone: