Authentication hook for demo

In #1385 (closed), we implemented a solution to force password change on the first login. It is a positive change, but it may interfere with a demo. Suppose you are forced to change the default password in the public demo; nobody else after the first user touching the demo will be able to access it until it is restarted.

We can load a dedicated authentication hook in the demo to accept fixed credentials or create a new account when the new credentials are provided. We can use the stork-hook-example hook as a base.