Add a role to install 'ntfy' push-notification server.

This commit adds a 'matrix-ntfy' role that runs Ntfy server in Docker with
simple configuration, and plumbing to add the role to the playbook.

TODO: documentation, self-check, database persistence.

roles/matrix-ntfy/tasks/init.yml

@@ -0,0 +1,5 @@
+---
+
+- set_fact:
+    matrix_systemd_services_list: "{{ matrix_systemd_services_list + ['matrix-ntfy.service'] }}"
+  when: matrix_ntfy_enabled|bool