backend: admin-interface: correctly use a flask-fixed session instead of a "normal" one
backend: admin-interface: correctly use a flask-fixed session instead of a "normal" one

Otherwise commiting to the session will fail.

walachey/football-worldcup-simulator-comp,walachey/football-worldcup-simulator,walachey/football-worldcup-simulator-comp,walachey/football-worldcup-simulator-comp,walachey/football-worldcup-simulator-comp,walachey/football-worldcup-simulator,walachey/football-worldcup-simulator,walachey/football-worldcup-simulator,walachey/football-worldcup-simulator