Chatterbox internal demo instance should be behind basic HTTP auth

[rxl881]

The Chatterbox instance at https://chatterbox-staging.element.io/demo.html is intended for internal testing. As a staging instance it could break at any point. We should put it behind HTTP basic auth, to avoid the URL being leaked to the public and it becoming relied upon, externally.

[uPvtkBlpiucvDUf]

You might want to delete that comment if you actually don't want this url to leak @rxl881