Set up buildbots to create nightly builds for all our packages

This is another potential sponsor F year 3 item that came out of a discussion with Roger on #tor-dev yesterday: can we have nightly builds for all our packages?

How far away are we from that? We had erinn working on #4459 (closed) to make nightly builds of the Windows bundles. We have Sebastian working on automating TBB builds and on creating an index page for buildbot statuses, right? What's left to do? And what else should we know before optimistically proposing this project to a sponsor?