Let feedback viewing penalty be configurable

[jpslav]

In certain research conditions students are required to view feedback in order to receive credit for having worked an exercise. When this option is set now, if a student does not view required feedback in the allotted window, they don't get any credit for the exercise. Some instructors have requested that the penalty for not viewing feedback be something less than losing 100% of the credit for an exercise.

This task is to make the penalty for not viewing feedback a configurable fraction between 0 and 1. A penalty of 1 results in a loss of 100% of the earned credit for an exercise. A penalty of .25 results in a loss of 25% of the earned credit for an exercise.

This will require changes to the FeedbackCondition model, views in app/views/feedback_conditions, and likely changes in the StudentExercise model (where the overall credit is calculated). Note also that there is a "score detail" view somewhere that should be updated to reflect the amount of the penalty (previously assumed to always be 100%).

I imagine that the views should show the penalty as a percent. The default value should be 100%, or you can not have a default and require that the user choose a value when requiring feedback viewing. Also, @kjdav should write a help topic for where the percent is entered (search for link_to_help to give @kjdav a placeholder in the code)

The developer should consider if the feedback penalty should be recorded in the research report (doesn't always apply so may not be great to do this; researchers can always get the information out by encoding the feedback percentage in the learning condition label).

cc @kjdav @Dantemss