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-bot-mjolnir/tasks/validate_config.yml

@@ -0,0 +1,9 @@
+---
+
+- name: Fail if required variables are undefined
+  ansible.builtin.fail:
+    msg: "The `{{ item }}` variable must be defined and have a non-null value."
+  with_items:
+    - "matrix_bot_mjolnir_access_token"
+    - "matrix_bot_mjolnir_management_room"
+  when: "vars[item] == '' or vars[item] is none"