Last seen indicator visual refresh

- Last seen indicator now spans the full conversation, with subtle
shadow highlights above and below
- Scrollbars now overlap the content of the conversation, allowing last
seen indicator to touch the right edge of the window.
- The iOS and Android conversation background is now #eee instead of
white, which meant that the outgoing messages (Android) and incoming
messages (iOS) had to be updated for contrast. They now have white
backgrounds.
- Similarly, the scroll down button needed more contrast, and its
background is now white in light themes.

FREEBIE

background.html

@@ -48,8 +48,10 @@
     </button>
   </script>
   <script type='text/x-tmpl-mustache' id='last-seen-indicator-view'>
-    <div class='text'>
-      {{ unreadMessages }}
+    <div class='bar'>
+      <div class='text'>
+        {{ unreadMessages }}
+      </div>
     </div>
   </script>
   <script type='text/x-tmpl-mustache' id='expired_alert'>