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/tasks/init.yml

@@ -1,3 +1,11 @@
+# If the matrix-synapse role is not used, `matrix_synapse_role_executed` won't exist.
+# We don't want to fail in such cases.
+- name: Fail if matrix-synapse role already executed
+  fail:
+    msg: >-
+      The matrix-bridge-appservice-discord role needs to execute before the matrix-synapse role.
+  when: "matrix_appservice_discord_enabled and matrix_synapse_role_executed|default(False)"
+
 - set_fact:
     matrix_systemd_services_list: "{{ matrix_systemd_services_list + ['matrix-appservice-discord'] }}"
   when: matrix_appservice_discord_enabled|bool
@@ -7,7 +15,7 @@
     matrix_synapse_container_extra_arguments: >
       {{ matrix_synapse_container_extra_arguments|default([]) }}
       +
-      {{ ["--mount type=bind,src={{ matrix_appservice_discord_base_path }}/discord-registration.yaml,dst=/matrix-appservice-discord-registration.yaml,ro"] }}
+      {{ ["--mount type=bind,src={{ matrix_appservice_discord_config_path }}/registration.yaml,dst=/matrix-appservice-discord-registration.yaml,ro"] }}
 
     matrix_synapse_app_service_config_files: >
       {{ matrix_synapse_app_service_config_files|default([]) }}