ui: add hint for problem_submit page

packages/ui-default/templates/problem_submit.html

@@ -7,7 +7,7 @@
       <div class="section__header">
         <h1 class="section__title">{{ _('Submit to Judge') }}</h1>
       </div>
-      <div class="section__body">
+      <div class="section__body typo" name="submit_section">
         <form method="post" enctype="multipart/form-data">
           <div class="hasjs--hide">
             {{ form.form_select({