Feedback penalty configuration should disallow '0' for clarity

[navilan]

Currently, we allow feedback penalty to be set to 0 and interpret it as "No penalty". For the sake of clarity, its better to disallow 0. /cc @kjdav

[Dantemss]

This is one of those things I would do differently: I would probably disallow null (in the model and database), remove the is_feedback_required_for_credit setting and just have 0 mean "off". Of course, no point in modifying all the code right now. Maybe in tutor 3.

[navilan]

@Dantemss - Agreed. That's a better design. My approach was to only add code. (is_feedback_required_for_credit) was already in use.