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.

docs/configuring-playbook-external-postgres.md

@@ -20,7 +20,7 @@ matrix_synapse_database_database: "your-postgres-server-database-name"
 
 # Rewire any other service (each `matrix-*` role) you may wish to use to use your external Postgres server.
 # Each service expects to have its own dedicated database on the Postgres server
-# and uses its own variable names (see `roles/matrix-*/defaults/main.yml) for configuring Postgres connectivity.
+# and uses its own variable names (see `roles/custom/matrix-*/defaults/main.yml) for configuring Postgres connectivity.
 ```
 
 The database (as specified in `matrix_synapse_database_database`) must exist and be accessible with the given credentials.