Countdown becomes Challenge

Finally decided to remove "Countdown" mode altogether and replace it with a (1 minute) "Challenge" that gives out several stars for doing exercises in the highest difficulty.

And there's also the Difficulty average to show which one you've solving. Most players are likely to prefer True/False, a.k.a. Difficulty 0) so this shows it.

I also remove the Policy link (it was next to the "Credits" button) to make space for a future one.

Edit: Seems like I forgot to update the version here, sorry about that.