Adapt admin user creation.

[caarmen]

Addresses issue #28.

Remove the code generated by chatgpt and create the logic from scratch.

Differences:

• Use logging instead of stdout.
• Read the admin username from the environment, instead of hardcoding it.
• Use the built-in UserManager.create_superuser() method: less stuff for us to manage.
• Log a warning in red when the admin user is created, inviting to change the admin user's password.