added a debug option to go-syncuser

* added an admin dashboard settings page for go-syncuser where we can set the debug option
* slog progress from go-syncuser's cron function when the debug option is set

see https://github.com/GigaOM/legacy-pro/issues/4020
Is this really what we want?

That is, do we want to update the option when we turn on debugging for a manual user sync?