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-common-after/tasks/main.yml

@@ -0,0 +1,19 @@
+---
+
+- ansible.builtin.import_tasks: "{{ role_path }}/tasks/start.yml"
+  when: run_start | bool
+  tags:
+    - start
+
+- ansible.builtin.import_tasks: "{{ role_path }}/tasks/stop.yml"
+  when: run_stop | bool
+  tags:
+    - stop
+
+- ansible.builtin.import_tasks: "{{ role_path }}/tasks/dump_runtime_results.yml"
+  tags:
+    - always
+
+- ansible.builtin.import_tasks: "{{ role_path }}/tasks/run_docker_prune.yml"
+  tags:
+    - run-docker-prune