Simplify the call model by handling upgrade in the webrtc model, and remove upgrade parameter from the webrtc model

This is intended as a first step for [this card](https://trello.com/c/Dmm17s5K). The idea is to start simplifying the call model and see how much we can remove some of the webrtc specifics from it. Dealing with upgrade is one of the parts that currently expects the webrtc model to have been handled by the text chat connection before the call model gets it - which is bleeding the lines between the models.

Hence, this moves handling the upgrade processing to the webrtc model.

As a side-effect, I've also discovered the upgrade parameter of the Offer message is actually redundant - it wasn't used anywhere on the receiving end of the upgrade by the chat models nor by the webrtc model.