MDL-44330 Assign: Assignment grading may not display expected student

Before this patch it was possible for the student displayed on the grading page to
not be the student that the user selected to grade. This would occur if:

1) The user had the table ordered by a value that could be modified,
   for example Last modified (submission), Grade, Last modified (grade)
2) Another user performed an action that was recorded in Moodle in the time
   between the user generating the table and clicking on a grade link.

If a user did not notice a different user had been loaded it could result in them giving
a grade to the incorrect user.

This patch ensures that the state of the table is cached every time it is viewed by a user
who has the capability to grade.

lib/tablelib.php

@@ -67,6 +67,7 @@ class flexible_table {
     var $column_suppress = array();
     var $column_nosort   = array('userpic');
     private $column_textsort = array();
+    /** @var boolean Stores if setup has already been called on this flixible table. */
     var $setup           = false;
     var $baseurl         = NULL;
     var $request         = array();