Don't send usernames across the data channel

Next part of the work for [this card](https://trello.com/c/ngI5QCux).

This removes the need to send usernames (and other unnecessary fields) across the data channel when doing text chat. Each peer already has the relevant information, and when we add in displaying of names, the information could leak to the other side, or the recipient mislead.

The next PR will replace the usernames with full names.