Move roles/matrix* to roles/custom/matrix*

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.

roles/custom/matrix-postgres/templates/sql/init-additional-db-user-and-role.sql.j2

@@ -0,0 +1,19 @@
+-- `CREATE USER` does not support `IF NOT EXISTS`, so we use this workaround to prevent an error and raise a notice instead.
+-- Seen here: https://stackoverflow.com/a/49858797
+DO $$
+BEGIN
+  CREATE USER "{{ additional_db.username }}";
+  EXCEPTION WHEN DUPLICATE_OBJECT THEN
+  RAISE NOTICE 'not creating user "{{ additional_db.username }}", since it already exists';
+END
+$$;
+
+-- This is useful for initial user creation (since we don't assign a password above) and for handling subsequent password changes
+-- TODO - we should escape quotes in the password.
+ALTER ROLE "{{ additional_db.username }}" PASSWORD '{{ additional_db.password }}';
+
+-- This will generate an error on subsequent execution
+CREATE DATABASE "{{ additional_db.name }}" WITH LC_CTYPE 'C' LC_COLLATE 'C' OWNER "{{ additional_db.username }}";
+
+-- This is useful for changing the database owner subsequently
+ALTER DATABASE "{{ additional_db.name }}" OWNER TO "{{ additional_db.username }}";