Make Discord bridge configuration playbook-managed

Well, `config.yaml` has been playbook-managed for a long time.
It's now extended to match the default sample config of the Discord
bridge.

With this patch, we also make `registration.yaml` playbook-managed,
which leads us to consistency with all other bridges.

Along with that, we introduce `./config` and `./data` separation,
like we do for the other bridges.

roles/matrix-bridge-appservice-discord/templates/systemd/matrix-appservice-discord.service.j2

@@ -25,11 +25,13 @@ ExecStart=/usr/bin/docker run --rm --name matrix-appservice-discord \
 			{% if matrix_appservice_discord_container_http_host_bind_port %}
 			-p {{ matrix_appservice_discord_container_http_host_bind_port }}:9005 \
 			{% endif %}
-			-v {{ matrix_appservice_discord_base_path }}:/data \
+			-v {{ matrix_appservice_discord_config_path }}:/cfg \
+			-v {{ matrix_appservice_discord_data_path }}:/data \
 			{% for arg in matrix_appservice_discord_container_extra_arguments %}
 			{{ arg }} \
 			{% endfor %}
-			{{ matrix_appservice_discord_docker_image }}
+			{{ matrix_appservice_discord_docker_image }} \
+			node /build/src/discordas.js -p 9005 -c /cfg/config.yaml -f /cfg/registration.yaml
 
 ExecStop=-/usr/bin/docker kill matrix-appservice-discord
 ExecStop=-/usr/bin/docker rm matrix-appservice-discord