* Remove increment behavior
* Dismiss when new messages arrive but the window is focused
* Update the indicator when window becomes focused.
// FREEBIE
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