minor adjustment to the session security to be merged

Signed-off-by: Spyros Gasteratos <morfeas3000@gmail.com>
Created an api for the challenges to interact with the backend

Each challenge-user pair has a unique id which allows the user to access the specific challenge
and identifies him for scoring.

Signed-off-by: Spyros Gasteratos <morfeas3000@gmail.com>