Add underscore to staff grading template

lms/templates/instructor/staff_grading.html

@@ -6,6 +6,7 @@
 <%block name="headextra">
 <%static:css group='style-course-vendor'/>
 <%static:css group='style-course'/>
+<script type="text/javascript" src="${static.url('js/vendor/underscore-min.js')}"></script>
 </%block>
 
 <%block name="title"><title>${_("{course_number} Staff Grading").format(course_number=course.display_number_with_default) | h}</title></%block>