Finalize demo app

[Cheaterman]

Everything in the title ; the demo app should allow users to anonymously authenticate (just giving a username) while the admin should be able to authenticate using username admin and a conf-defined password, making them able to edit/remove messages.

That also means messages should have an author. Additionally, also give them a creation date ; the icing on the cake would be to use SSEs to instantly and transparently dispatch new messages to all connected clients.

EDIT: The interest of adding an admin user is two-fold - forces us to set a SECRET_KEY which is going to be useful for most projects anyway, as well as demonstrating zero-knowledge-auth.