MDL-49637 assign: Workflow & blind marking do not always show feedback

Before this patch if an assignment has workflow and blind marking enabled if a teacher
sets a submission to have its grades released to students before they reveal student
identities the feedback they have given is not displayed to the student.

The patch causes the reveal_identities() method to pass the correct value to the
get_feedback_plugin_by_type() function in the assign class.

mod/assign/locallib.php

@@ -5493,6 +5493,7 @@ class assign {
 
         $adminconfig = $this->get_admin_config();
         $gradebookplugin = $adminconfig->feedback_plugin_for_gradebook;
+        $gradebookplugin = str_replace('assignfeedback_', '', $gradebookplugin);
         $grades = $DB->get_records('assign_grades', array('assignment'=>$this->get_instance()->id));
 
         $plugin = $this->get_feedback_plugin_by_type($gradebookplugin);