Add elwig-backend.service to .deb package

pkg/postinst

@@ -0,0 +1,25 @@
+#!/bin/sh
+set -e
+if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ] || [ "$1" = "abort-deconfigure" ] || [ "$1" = "abort-remove" ] ; then
+  # This will only remove masks created by d-s-h on package removal.
+  deb-systemd-helper unmask 'elwig-backend.service' >/dev/null || true
+  # was-enabled defaults to true, so new installations run enable.
+  if deb-systemd-helper --quiet was-enabled 'elwig-backend.service'; then
+    # Enables the unit on first installation, creates new
+    # symlinks on upgrades if the unit file has changed.
+    deb-systemd-helper enable 'elwig-backend.service' >/dev/null || true
+  else
+    # Update the statefile to add new symlinks (if any), which need to be
+    # cleaned up on purge. Also remove old symlinks.
+    deb-systemd-helper update-state 'elwig-backend.service' >/dev/null || true
+  fi
+  if [ -d /run/systemd/system ]; then
+    systemctl --system daemon-reload >/dev/null || true
+    if [ -n "$2" ]; then
+      _dh_action=restart
+    else
+      _dh_action=start
+    fi
+    deb-systemd-invoke $_dh_action 'elwig-backend.service' >/dev/null || true
+  fi
+fi