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
code sniffer happiness
* separated the debug option setter and getter into two different functions to minimize confusion (debug() and set_debug())
* by default do not persist the debug option unless the user sets the $persist parameter in set_debug()
* use wp_die, wp_send_json_error and wp_send_json_success in set_debug_ajax().
removed the unused 'go_syncuser_user_update_sync' admin ajax
added nonce and nonce-verification when submitting the debug option setting
use plugins_url() correctly and not do that concat silliness!
Merge pull request #5 from wluo/github-4020

added a debug option to go-syncuser