Assign difficulties to exercises

[ErikSchierboom]

The exercises currently all have the same difficulty.
We should make an attempt to assign "proper" difficulties to them, which range from 1 (easiest) to 10 (hardest)

[kytrinyx]

Could we make a rough guess using mean values from other tracks? Or is Prolog different enough that it would be a bad guess?

[ErikSchierboom]

I think so. We could maybe use the build page's success rating for some estimates, but that wouldn't work for newer exercises. I also don't mind it being somewhat incorrect. Having difficulties will be better than having none.