remove withdrawn predictions from the unjudged predictions list
1. Move the *not withdrawn* checking logic from the users controller to the prediction model.

   In the users controller, the `due_for_judgement` method's final step is:

     @predictions = @predictions.select { |x| x.due_for_judgement? }

   which triggers the `due_for_judgement?` method in the prediction model.

   A withdrawn prediction is surely not to be judged, so *not withdrawn* checking should be in the `due_for_judgement?` model method. This makes it more consistent and easier to be tested.

2. Add test case for the above change

[#71639008]
Merge branch 'foxbenjaminfox-unjudged'