Add a contact add/remove button in chat.

For single chat sessions and those that are 'users' add a contact add/remove star to the chat menu next to call, upload file, and geolocation.

![chat-contact](https://cloud.githubusercontent.com/assets/4820641/4422348/282e24b4-458c-11e4-9fc2-a2311303f16f.png)

These handlers need to be removed when the subscope is destroyed in `killRoom()`. Bad enough that we're adding potentially quite a few event handlers, but far worse to leak them...