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-synapse/templates/synapse/usr-local-bin/matrix-synapse-register-user.j2

@@ -0,0 +1,17 @@
+#jinja2: lstrip_blocks: "True"
+#!/bin/bash
+
+if [ $# -ne 3 ]; then
+	echo "Usage: "$0" <username> <password> <admin access: 0 or 1>"
+	exit 1
+fi
+
+user=$1
+password=$2
+admin=$3
+
+if [ "$admin" -eq "1" ]; then
+	docker exec matrix-synapse register_new_matrix_user -u "$user" -p "$password" -c /data/homeserver.yaml --admin http://localhost:{{ matrix_synapse_container_client_api_port }}
+else
+	docker exec matrix-synapse register_new_matrix_user -u "$user" -p "$password" -c /data/homeserver.yaml --no-admin http://localhost:{{ matrix_synapse_container_client_api_port }}
+fi