Add registered decorator

[BaboucheOne]

We need decorators to ensure that the student can access certains routes. For example :

@registered
def speak_to_bot_when_registered(self):
      pass

@unregistered
def speak_to_bot_when_unregistered(self):
      pass

We can only call the method speak_to_bot_when_registered when the student is registered. The decorator will handle the logic for us.

[BaboucheOne]

Why not using has_role decorator ?