Add the user to `systemd-journal` group by default so that `journalctl --user` works

Currently `journalctl --user` does not work in the default setup:
```console
$ journalctl  --user
Hint: You are currently not seeing messages from the system.
      Users in groups 'adm', 'systemd-journal' can see all messages.
      Pass -q to turn off this notice.
No journal files were opened due to insufficient permissions.
```

Note: non-systemd distributions do not have `systemd-journal`