accessibility: Label messages region, and add aria-live.

This tells assistive technologies to automatically speak new messages
when they arrive.

Author: ndarilek

templates/zerver/app/home.html

@@ -124,9 +124,9 @@ <h4>{{ _("You haven't been mentioned yet!") }}</h4>
     <div id="empty_search_narrow_message" class="empty_feed_notice">
         <h4>{{ _('Nobody has talked about that yet!') }}</h4>
     </div>
-    <div class="message_table focused_table" id="zhome">
+    <div class="message_table focused_table" id="zhome" role="region" aria-live="polite" aria-label="{{ _('Messages') }}">
     </div>
-    <div class="message_table" id="zfilt">
+    <div class="message_table" id="zfilt" role="region" aria-live="polite" aria-label="{{ _('Messages') }}">
     </div>
     <div id="typing_notifications">
     </div>