🚀(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/desk/values.yaml

@@ -30,7 +30,7 @@ commonEnvVars: &commonEnvVars
 ## @param ingress.host Host for the Ingress
 ## @param ingress.path Path to use for the Ingress
 ingress:
-  enabled: true
+  enabled: false
   className: null
   host: desk.example.com
   path: /