Use caption instead of tr to label the answers table

We might change the style of this caption or remove it completely in the
future. In the meantime, we use the right HTML tag for the task to give
information regarding what the table is about, and that tag is
`<caption>`.

app/views/admin/poll/questions/show.html.erb

@@ -35,12 +35,8 @@
 </div>
 
 <table class="margin-top">
-  <tr>
-    <th colspan="5" scope="col" class="with-button">
-      <%= t("admin.questions.show.valid_answers") %>
-    </th>
-  </tr>
-
+  <caption><%= t("admin.questions.show.valid_answers") %></caption>
+  <thead>
     <tr>
       <th><%= t("admin.questions.show.answers.title") %></th>
       <th scope="col" class="medium-7"><%= t("admin.questions.show.answers.description") %></th>
@@ -48,6 +44,7 @@
       <th scope="col" class="text-center"><%= t("admin.questions.show.answers.documents") %></th>
       <th scope="col" class="text-center"><%= t("admin.questions.show.answers.videos") %></th>
     </tr>
+  </thead>
 
   <tbody class="sortable" data-js-url="<%= admin_question_answers_order_answers_path(@question.id) %>">
     <% @question.question_answers.each do |answer| %>