🚀(helm) prepare staging deployment

Thx @rouja for your help on deploying Desk. This commit slightly modifies
helm charts and helmfile to prepare the initial project deployment in a
staging environment.

@rouja updates:
- added secrets files for dev and staging environments (dev's one is empty)
- disable ingress by default, to avoid any security issue
- added an extra chart to benefit from Indie hoster Postgres operator

Thx to this commit we deployed a first draft version figured out
that the Django session were broken. We are using a cache session engine,
and wrongly configure cache backend to local memory. Thus, Django server
is not able to resolve the session, and enters in an infinite loop to
log-in the user.

src/helm/extra/Chart.yaml

@@ -0,0 +1,5 @@
+apiVersion: v2
+name: extra
+description: A Helm chart to add some manifests to desk
+type: application
+version: 0.1.0

src/helm/extra/templates/postgresql.yaml

@@ -0,0 +1,7 @@
+apiVersion: core.libre.sh/v1alpha1
+kind: Postgres
+metadata:
+  name: postgresql
+  namespace: {{ .Release.Namespace | quote }}
+spec:
+  database: desk