Switch to Debian 10 and Ansible

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.

ansible/roles/nodejs/tasks/main.yml

@@ -0,0 +1,20 @@
+---
+- name: Add APT key for NodeSource Node.js repository
+  apt_key:
+    url: https://deb.nodesource.com/gpgkey/nodesource.gpg.key
+
+- name: Add NodeSource Node.js APT repository
+  apt_repository:
+    repo: deb https://deb.nodesource.com/node_14.x buster main
+
+- name: Install Node.js
+  apt:
+    name: nodejs
+
+- name: Install npm packages defined in package.json
+  become_user: "{{ app_username }}"
+  community.general.npm:
+    path: "{{ app_dir }}"
+    # --no-bin-links option is needed to prevent npm from creating symlinks in the .bin
+    # directory, which doesn't work inside Vagrant on Windows
+    no_bin_links: true