Add new Last Seen Indicator with unread count, scroll to it

This is to ensure that when there are a lot of unread messages, the user
is given the chance to see all of them by being scrolled to the oldest
new message.

When a new message comes in, the indicator will be incremented.

When the user sends a message or switches away from the conversation,
the last seen indicator will be removed.

FREEBIE

background.html

@@ -42,6 +42,11 @@
       </div>
     </div>
   </script>
+  <script type='text/x-tmpl-mustache' id='last-seen-indicator-view'>
+    <div class='text'>
+      {{ unreadMessages }}
+    </div>
+  </script>
   <script type='text/x-tmpl-mustache' id='expired_alert'>
     <a target='_blank' href='https://chrome.google.com/webstore/detail/bikioccmkafdpakkkcpdbppfkghcmihk'>
       <button class='upgrade'>{{ upgrade }}</button>
@@ -630,6 +635,7 @@
   <script type='text/javascript' src='js/emoji_util.js'></script>
 
   <script type='text/javascript' src='js/views/whisper_view.js'></script>
+  <script type='text/javascript' src='js/views/last_seen_indicator_view.js'></script>
   <script type='text/javascript' src='js/views/debug_log_view.js'></script>
   <script type='text/javascript' src='js/views/toast_view.js'></script>
   <script type='text/javascript' src='js/views/attachment_preview_view.js'></script>