Add Helm chart (#14090)

* add Helm chart

known issues/future work:

- SSO is unsupported

- S3/Minio/GCS is unsupported

- Swift is unsupported

- WEB_DOMAIN is unsupported

- Tor is unsupported

* helm: clarify how LOCAL_DOMAIN is set

* helm: add chart description

* helm: make DB_POOL and Sidekiq concurrency configurable

* helm: only enforce pod affinity when using ReadWriteOnce

* helm: clarify compatibility

* helm: clean up application variables

* helm: add job to create initial admin

chart/templates/pvc-system.yaml

@@ -0,0 +1,13 @@
+---
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: {{ template "mastodon.fullname" . }}-system
+  labels:
+    {{- include "mastodon.labels" . | nindent 4 }}
+spec:
+  accessModes:
+    - {{ .Values.persistence.system.accessMode }}
+  resources:
+    {{- toYaml .Values.persistence.system.resources | nindent 4}}
+  storageClassName: {{ .Values.persistence.system.storageClassName }}