Implement /user/@userid:domain?action=chat
This is a URL that can be used to start a chat with a user.
- If the user is a guest, setMxId dialog will appear before anything and a defered action will cause `ChatCreateOrReuseDialog` to appear once they've logged in.
- If the user is registered, they will not see the setMxId dialog.
fixes https://github.com/vector-im/riot-web/issues/4034
