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-heisenbridge.md

@@ -8,7 +8,7 @@ See the project's [README](https://github.com/hifi/heisenbridge/blob/master/READ
 
 ## Configuration
 
-Below are the common configuration options that you may want to set, exhaustive list is in [the bridge's defaults var file](../roles/matrix-bridge-heisenbridge/defaults/main.yml).
+Below are the common configuration options that you may want to set, exhaustive list is in [the bridge's defaults var file](../roles/custom/matrix-bridge-heisenbridge/defaults/main.yml).
 
 At a minimum, you only need to enable the bridge to get it up and running (`inventory/host_vars/matrix.DOMAIN/vars.yml`):