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
There's a few, possible incompatible things going on here:

1. A getter for the option
1. The ability to temporarily override the option, with no intention of persisting that in the database or incurring the cost and risk of setting then reverting it
1. The ability to set the option

Perhaps this should be broken up into separate setters and getters to eliminate the confusion and complexity?