add fade out effect to predictions

As time from last known, increases, the confidence aka opacity, decreases.  By
11 minutes, the prediction starts to aggresively deviate towards means in ways
that are not believable, as Adrien de Croy points out.  This makes the
prediction start to fade out by 12 minutes in.  By 30 minutes out, the
prediction is invisible, and requires straining to see after 15 minutes.