Allow adding monitors when reporting an issue

By adding monitors to the $g_bug_report_page_fields array, users
having $g_monitor_add_others_bug_threshold access level will be able
to select users to add to the issue's monitoring list.

Monitors will only be notified of the submission if both their e-mail
prefs and the flags allow it ($g_notify_flags['new']['monitor'] = ON).

Fixes 0006128