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
I'd `esc_attr` this nonce because it would make it more obvious that it was escaped, but I understand that it is in fact safe since it is set from the return of `wp_create_nonce` right before calling this template.