Track unread chat messages across chat sessions

Depends on https://github.com/dimagi/dimagi-utils/pull/177

Upon opening the chat window, shows the user the sms that have been sent or received for the contact since the last time the user had the chat window open for that contact. Also adds the ability to count a message as read once anyone has seen it rather than on an individual user basis.

![chat_window](https://f.cloud.github.com/assets/1028814/2073300/ddf94d2c-8d55-11e3-9f86-99e5e038c93e.png)