Provide credentials for a know user

[johan-smits]

Describe the solution you'd like
Currently all chat users are "new" users. With the application token new users are created. In our environment we have all our clients registered in Matrix. By making the config.json endpoint dynamic (should send cookie credentials retrieving it) we could provide a predefined session token in the config for the known user. This helps us to identify the user (#63) and detect the preferred language for our bot.