ntfy: Migrate to Kustomize

Using Kustomize, we can define the configuration file separately from
the Kubernetes resources, and use `configMapGenerators` to generate the
ConfigMap for it.  Additionally, this will make it possible to update
_ntfy_ using `updatebot`.

ntfy/server.yml

@@ -0,0 +1,6 @@
+base-url: https://ntfy.pyrocufflink.net
+behind-proxy: true
+listen-http: '[::]:2586'
+attachment-cache-dir: /var/cache/ntfy/attachments
+attachment-file-size-limit: 100M
+enable-metrics: true