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
Typically we have done this instead of concatenating:
```
plugins_url( '/js/go-usersync-admin-settings.js', __FILE__ )
```