Improve chat window user id handling

This fixes a bunch of issues around the chat window and obtaining the user's id. The user id is now received from `talkilla.login-success` from the worker, as soon as the port is created.

This means that we no longer have to store the user's id in the call model, and in the `TextChatView` separately, but we can use the `app.data.user` information.

This fixes comments from a previous PR and this card: https://trello.com/c/artOTTWH

Note that we may move `app.data.user` to something else later, but I have kept it the same as the sidebar for now as that will ensure consistency.