MDL-46721 assign: Make blind marking participant numbers match

Previously emails were setting the "From" header to the incorrect
participant number.

mod/assign/locallib.php

@@ -4990,11 +4990,9 @@ class assign {
      * @param int $updatetime
      * @return void
      */
-    public function send_notification($userfrom,
-                                      $userto,
-                                      $messagetype,
-                                      $eventtype,
-                                      $updatetime) {
+    public function send_notification($userfrom, $userto, $messagetype, $eventtype, $updatetime) {
+        global $USER;
+        $uniqueid = $this->get_uniqueid_for_user(core_user::is_real_user($userfrom->id) ? $userfrom->id : $USER->id);
         self::send_assignment_notification($userfrom,
                                            $userto,
                                            $messagetype,
@@ -5006,7 +5004,7 @@ class assign {
                                            $this->get_module_name(),
                                            $this->get_instance()->name,
                                            $this->is_blind_marking(),
-                                           $this->get_uniqueid_for_user($userfrom->id));
+                                           $uniqueid);
     }
 
     /**