more debug slogs among other things

* use the new go-syncuser()->debug() call and slog extra info when debug is on
* added nonce and corresponding check to the "Sync to Marketo" button
* use `wp_die()` instead of `die`

depends on https://github.com/GigaOM/go-syncuser/pull/5

see https://github.com/GigaOM/legacy-pro/issues/4020