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-bridge-mautrix-facebook.md

@@ -39,7 +39,7 @@ matrix_mautrix_facebook_configuration_extension_yaml: |
       default: true
 ```
 
-You may wish to look at `roles/matrix-bridge-mautrix-facebook/templates/config.yaml.j2` and `roles/matrix-bridge-mautrix-facebook/defaults/main.yml` to find other things you would like to configure.
+You may wish to look at `roles/custom/matrix-bridge-mautrix-facebook/templates/config.yaml.j2` and `roles/custom/matrix-bridge-mautrix-facebook/defaults/main.yml` to find other things you would like to configure.
 
 
 ## Set up Double Puppeting