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

stylesheets/_android.scss

@@ -28,7 +28,7 @@
   }
 
   .outgoing .bubble {
-    background-color: $grey_l;
+    background-color: white;
   }
   .outgoing .hourglass {
     @include hourglass(#999);