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/postgresql_plpython/meta/main.yml

@@ -0,0 +1,3 @@
+---
+dependencies:
+  - role: postgresql

ansible/roles/postgresql_plpython/tasks/main.yml

@@ -0,0 +1,12 @@
+---
+- name: Install PL/Python3 procedural language for PostgreSQL
+  apt:
+    name: postgresql-plpython3-{{ postgresql_version }}
+
+- name: Set PYTHONPATH env var for PostgreSQL
+  lineinfile:
+    path: /etc/postgresql/{{ postgresql_version }}/main/environment
+    regexp: "^PYTHONPATH="
+    line: "PYTHONPATH='{{ venv_dir }}/lib/python{{ python_version }}/site-packages:{{ app_dir }}'"
+  notify:
+    - Restart postgresql