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/ntfy.yaml

@@ -5,25 +5,6 @@ metadata:
   labels:
     app.kubernetes.io/instance: ntfy
 
----
-apiVersion: v1
-kind: ConfigMap
-metadata:
-  name: ntfy
-  namespace: ntfy
-  labels:
-    app.kubernetes.io/name: ntfy
-    app.kubernetes.io/component: ntfy
-    app.kubernetes.io/instance: ntfy
-    app.kubernetes.io/part-of: ntfy
-data:
-  server.yml: |+
-    base-url: https://ntfy.pyrocufflink.net
-    behind-proxy: true
-    listen-http: '[::]:2586'
-    attachment-cache-dir: /var/cache/ntfy/attachments
-    attachment-file-size-limit: 100M
-
 ---
 apiVersion: v1
 kind: Service