Provide credentials for a know user

**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.