Increase the final comment textarea size

The comment textarea size has been increase to 1000 and the final
comment size is smaller now. Increases to 1000 to match the comment
textarea size. Reference: https://github.com/edx/edx-ora2/pull/811

openassessment/templates/openassessmentblock/oa_rubric.html

@@ -69,7 +69,7 @@
                     id="{{ rubric_type }}__assessment__rubric__question--feedback__value"
                     class="assessment__rubric__question--feedback__value"
                     placeholder="{{ rubric_feedback_default_text }}"
-                    maxlength="500"
+                    maxlength="1000"
                 >
                 </textarea>
             </div>