Accessibility: Turned discussion edit button into button for SR users

Fixes CNVS-13971

Test plan:
- Visit a discussion with edit privileges
- Using a screen reader, verify that the edit button is reported as being a
  button

Change-Id: Ie89a3595d33f14fbb990dc2f8140216d5a08bfe2
Reviewed-on: https://gerrit.instructure.com/38469
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jayce Higgins <jhiggins@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: David Josse <david@instructure.com>

app/views/discussion_topics/show.html.erb

@@ -65,8 +65,8 @@
             <a href="<%= context_url(@topic.context,
                                      :edit_context_discussion_topic_url,
                                      @topic) %>"
-               class="btn edit-btn">
-                 <i class="icon-edit" aria-hidden="hidden"></i>
+               class="btn edit-btn" role="button">
+                 <i class="icon-edit" aria-hidden="true"></i>
                  <%= t(:edit, 'Edit') %>
             </a>
           <% end %>