Pre-install Job does not accept connection string from secret

Currently, the only way to configure secrets for the Pre-install Job is by setting .Values.env.secret.*. This is not perfect, since values.yaml often contain configuration we want to check into git, while the encryption keys and the full connection string certainly not. I would expect the Job to accept secret ref to get its secrets.

This is already done in all the deployment files (!6 (merged)):

{{- if .Values.existingSecret }}
    - secretRef:
      name: {{ .Values.existingSecret }}
{{- end }}

I propose to add .Values.existingSecret as a secretRef to the Pre-install Job.

Potentially also

{{- if .Values.postgresql.enabled }}
    - name: DATABASE_PASSWORD
      valueFrom:
          secretKeyRef:
              name: {{ default (include "django.postgresql.fullname" .) .Values.postgresql.auth.existingSecret }}
              key: postgres-password
{{- end }}

I am not fluent in this project enough to be sure how this works.