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/tasks/validate_config.yml

@@ -0,0 +1,39 @@
+---
+
+- name: (Deprecation) Warn about matrix_postgres_use_external usage
+  ansible.builtin.fail:
+    msg: >
+      The `matrix_postgres_use_external` variable defined in your configuration is not used by this playbook anymore!
+      You'll need to adapt to the new way of using an external Postgres server.
+      It's a combination of `matrix_postgres_enabled: false` and specifying Postgres connection
+      details in a few `matrix_postgres_connection_` variables.
+      See the "Using an external PostgreSQL server (optional)" documentation page.
+  when: "'matrix_postgres_use_external' in vars"
+
+# This is separate (from the other required variables below),
+# because we'd like to have a friendlier message for our existing users.
+- name: Fail if matrix_postgres_connection_password not defined
+  ansible.builtin.fail:
+    msg: >-
+      The playbook no longer has a default Postgres password defined in the `matrix_postgres_connection_password` variable, among lots of other Postgres changes.
+      You need to perform multiple manual steps to resolve this.
+      See our changelog for more details:
+      https://github.com/spantaleev/matrix-docker-ansible-deploy/blob/master/CHANGELOG.md#breaking-change-postgres-changes-that-require-manual-intervention
+  when: "matrix_postgres_connection_password == ''"
+
+- name: Fail if required Postgres settings not defined
+  ansible.builtin.fail:
+    msg: >-
+      You need to define a required configuration setting (`{{ item }}`).
+  when: "vars[item] == ''"
+  with_items:
+    - "matrix_postgres_connection_hostname"
+    - "matrix_postgres_connection_port"
+    - "matrix_postgres_connection_username"
+    - "matrix_postgres_connection_password"
+    - "matrix_postgres_db_name"
+
+- name: Fail if Postgres password length exceeded
+  ansible.builtin.fail:
+    msg: "The maximum `matrix_postgres_connection_password` length is 99 characters"
+  when: "matrix_postgres_connection_password | length > 99"