Fix accessibility issues for the rubric grading form in the forum grading UI

Axe reports two accessibility issues for the marking guide grading form template for the forum grading UI.

1. Empty button for the criterion description. e.g.

   <button class="btn btn-link px-1" aria-controls="gradingform_guide-7-criteria-5-description" aria-expanded="false" data-target="#gradingform_guide-7-criteria-5-description" data-toggle="collapse" type="button">

2. Empty button description for the add frequently used comment button. e.g.

   <button class="btn btn-icon form-inset-item icon-no-margin p-0 mt-1 mr-1 text-reset" aria-controls="gradingform_guide-7-criteria-5-remark-frequent-comments" aria-expanded="true" data-toggle="collapse" data-target="#gradingform_guide-7-criteria-5-remark-frequent-comments" type="button" id="yui_3_17_2_1_1586240583780_992">