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.