[Reviewed] Gradecraft Issue 270

Update predictor sliders to have fixed increments that match assignment score_levels

This is my first pull request so I expect some issues here and there. I'm still getting the hang of javascript so I'm sure there's better things than what I did, but overall I think I hit a lot of the use cases and more from what we were going for.

One thing to look at would be the "step controversy" I kind of enter below -- I'm sure there will be some disagreement or adjustments there as we figure out a good solution.