Improve accessibility of forum nav topic menu

Add the aria-haspopup attribute and additional explanatory text for
screen reader users.

CHANGELOG.rst

@@ -5,7 +5,7 @@ These are notable changes in edx-platform.  This is a rolling list of changes,
 in roughly chronological order, most recent first.  Add your entries at or near
 the top.  Include a label indicating the component affected.
 
-LMS: Improved accessibility of forum navigation sort buttons
+LMS: Improved accessibility of parts of forum navigation sidebar
 
 LMS: enhanced shib support, including detection of linked shib account
 at login page and support for the ?next= GET parameter.

lms/templates/discussion/_thread_list_template.html

@@ -12,7 +12,11 @@
                 <i class="icon icon-reorder"></i>
                 <span class="sr">${_("Discussion Topics")}</span>
             </a>
-            <a href="#" class="browse-topic-drop-btn"><span class="current-board">${_("Show All Discussions")}</span> <span class="drop-arrow">▾</span></a>
+            <a href="#" class="browse-topic-drop-btn" aria-haspopup="true" aria-owns="browse-topic-drop-menu">
+                <span class="sr">${_("Discussion topics; current selection is: ")}</span>
+                <span class="current-board">${_("Show All Discussions")}</span>
+                <span class="drop-arrow" aria-hidden="true">▾</span>
+            </a>
         </div>
         <%include file="_filter_dropdown.html" />
         <div class="search">