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.

setup.yml

@@ -60,6 +60,7 @@
     - matrix-etherpad
     - matrix-email2matrix
     - matrix-sygnal
+    - matrix-ntfy
     - matrix-nginx-proxy
     - matrix-coturn
     - matrix-aux