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

stylesheets/_android.scss

@@ -70,4 +70,8 @@
   .inactive button.back {
     @include header-icon-black('/images/back.svg');
   }
+
+  .message-list .last-seen-indicator-view .text {
+    margin-top: 2em;
+  }
 }