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-user-creator/tasks/main.yml

@@ -0,0 +1,9 @@
+---
+
+- when: matrix_user_creator_users | length > 0
+  ansible.builtin.import_tasks: "{{ role_path }}/tasks/setup.yml"
+  tags:
+    # This role intentionally doesn't do work on a `setup-all` tag.
+    # If it did, the initial installation (`--tags=setup-all`) would also potentially polute the database with data,
+    # which would make importing a database dump problematic.
+    - ensure-matrix-users-created