MDL-35726: Use a different form identifier for each form when using "Save and next"

This prevents a bug where the next student feedback will default to the values that
were on the last submitted form. This is a similar case to having multiple instances
of the same form on the same page - each one needs a unique identifier to distinguish
between them. In this case I include the $rownum which changes as you go to each student
in the list.

mod/assign/gradeform.php

@@ -59,6 +59,17 @@ class mod_assign_grade_form extends moodleform {
         }
     }
 
+    /**
+     * This is required so when using "Save and next", each form is not defaulted to the previous form.
+     * Giving each form a unique identitifer is enough to prevent this (include the rownum in the form name).
+     *
+     * @return string - The unique identifier for this form.
+     */
+    protected function get_form_identifier() {
+        $params = $this->_customdata[2];
+        return get_class($this) . '_' . $params['rownum'];
+    }
+
     /**
      * Perform minimal validation on the grade form
      * @param array $data