This changes the site to run on Debian 10 instead of Ubuntu 16.04. It
also fully converts the previous Salt setup to use Ansible instead.
Most of this was a relatively straightforward conversion, and it should
be very close to equivalent. One notable difference is that I removed
the setup for the "monitoring" server, since I wasn't confident that the
way of setting up self-hosted Sentry and Grafana was working any more.
I'll look to re-add that at some point, but it's not urgent.
Boussole (which watches the SCSS files for changes and compiles them)
has just been running in the same venv as the main app until now, but
it's holding back the version of the click package. There's no real
reason that it needs to be in the app venv, so this moves it to its own
one, which also eliminates quite a few other packages that were only
being installed because of Boussole.