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.

examples/vars.yml

@@ -13,7 +13,7 @@ matrix_domain: YOUR_BARE_DOMAIN_NAME_HERE
 
 # The Matrix homeserver software to install.
 # See:
-#  - `roles/matrix-base/defaults/main.yml` for valid options
+#  - `roles/custom/matrix-base/defaults/main.yml` for valid options
 # - the `docs/configuring-playbook-IMPLEMENTATION_NAME.md` documentation page, if one is available for your implementation choice
 matrix_homeserver_implementation: synapse