MDL-38525: Assignment upgrade - Error with comments for unenrolled students.

Students with comments on 'upload' assignments cause errors for the upgrade
tool if they are no longer enrolled. The solution is to allow siteadmin to
see student submissions even if the student is not currently enrolled (you
must be siteadmin to run the upgrade tool).

mod/assign/locallib.php

@@ -2061,6 +2061,9 @@ class assign {
     public function can_view_submission($userid) {
         global $USER;
 
+        if (is_siteadmin()) {
+            return true;
+        }
         if (!is_enrolled($this->get_course_context(), $userid)) {
             return false;
         }