CI & auto-deploy

Plan:

set up a test deployment location at test.kwant-project.org.
set up restricted rsync users with passwordless keys that can only copy to
- Test website
- Actual website
set up CI to build the website and deploy
- To test location on any push
- To the main location on pushes to master

The keys can be stored on the computer providing runners and mounted at .ssh/id_rsa as volumes. The runner for the test location may or may not be shared with other website users (mildly prone to abuse). @cwg, @jbweston – opinion about this last point?