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
yup this action was added when we thought `go-syncuser` would trigger a global sync but then we realized it's better for each service to have its own sync button, so this action isn't needed any more.