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.
This won't affect requests for static files or anything except ones that
get proxied to the app.
The current configuration is based on IP, and allows a rate of 4/sec,
with an additional burst of 5 above the limit permitted, and burst
requests allowed to go through immediately (nodelay). For more info:
https://www.nginx.com/blog/rate-limiting-nginx/