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-cactus-comments/templates/env.j2

@@ -0,0 +1,6 @@
+CACTUS_HS_TOKEN={{ matrix_cactus_comments_hs_token }}
+CACTUS_AS_TOKEN={{ matrix_cactus_comments_as_token }}
+CACTUS_HOMESERVER_URL={{ matrix_cactus_comments_homeserver_url }}
+CACTUS_USER_ID=@{{ matrix_cactus_comments_user_id }}:{{ matrix_domain }}
+
+{{ matrix_cactus_comments_environment_variables_extension }}