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_tildes_dbs/tasks/database.yml

@@ -0,0 +1,38 @@
+---
+- name: Create database and enable access and all necessary extensions
+  become_user: postgres
+  block:
+    - name: Create database
+      community.postgresql.postgresql_db:
+        name: "{{ item }}"
+        owner: tildes
+
+    - name: Enable extensions
+      community.postgresql.postgresql_ext:
+        name: "{{ extension }}"
+        db: "{{ item }}"
+      loop: "{{ extensions }}"
+      # since the "outer" loop by include_tasks is already using the `item` variable,
+      # this will use `extension` for the loop inside here
+      loop_control:
+        loop_var: extension
+
+    - name: Add database to pg_hba.conf
+      community.postgresql.postgresql_pg_hba:
+        dest: /etc/postgresql/{{ postgresql_version }}/main/pg_hba.conf
+        databases: "{{ item }}"
+        users: tildes
+        contype: local
+        method: trust
+      notify:
+        - Reload postgresql
+
+    - name: Add database to pgbouncer.ini
+      become_user: root
+      lineinfile:
+        path: /etc/pgbouncer/pgbouncer.ini
+        line: "{{ item }} ="
+        insertafter: "^\\[databases\\]$"
+        firstmatch: true
+      notify:
+        - Reload pgbouncer