helm: add env variable for session inactivity to server

Adds an env variable that contains the maximum number of days for which
an interactive session can stay inactive before being automatically
closed.

Closes reanahub/reana-client#657

helm/reana/templates/reana-server.yaml

@@ -86,6 +86,8 @@ spec:
           {{- end }}
           - name: REANA_KUBERNETES_JOBS_MAX_USER_TIMEOUT_LIMIT
             value: !!str {{ .Values.kubernetes_jobs_max_user_timeout_limit | default 1209600 }}  # 1209600 seconds = 14 days
+          - name: REANA_INTERACTIVE_SESSION_MAX_INACTIVITY_PERIOD
+            value: {{ .Values.interactive_sessions.maximum_inactivity_period | default "forever" | quote }}
           - name: REANA_KUBERNETES_JOBS_TIMEOUT_LIMIT
             value: !!str {{ .Values.kubernetes_jobs_timeout_limit | default 604800 }}  # 604800 seconds = 7 days
           - name: REANA_KUBERNETES_JOBS_MEMORY_LIMIT