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
Looks like the answer is yes, this is not needed: https://github.com/GigaOM/go-mailchimp/pull/8/files#r16487583