Improving the code of the GP end Cutscenes

My plan was to add a button where you can save a random GP if you liked playing it, but I finally took the redundant code from the two possible GP end scenes and gave it to a new base class. I also removed many unnecessary lines and added a deactivated save button which wouldn't do anything. I'll finish the button in another pull request.
This does not belong in the "else"; this is the "else" that goes with "if (challenge won)".
Placing this code in the else means that if you win the challenge, the "save GP" button will be visible even if it's not a random GP