🏗️🛂 Authentication facade

In the best case, we should allow multiple methods of authentication (@see #12 #13 #14 #15).

As soon as at least two of those methods are implemented, we should provide a facade, to:

1. abstract authentication from Hecate App POV, thus making code more maintenable
2. allow authentication to be pluginizable, so our code is more modular and others can develop their own authentication methods without bothering us

This issue covers three tasks:

• designing the interface code architecture
• refactoring javascript codes to use the design
• implementing a (HTML) page allowing to chose authentication method