lml/ost#346: Make feedback penalty configurable.

- Add feedback penalty field to the settings store.
- Add feedback penalty to show and edit views.
- Add JS to show / hide penalty based on feedback required.
- Add help file for @kjdav with some help text.
- Ensure score detail page does not need any updates.
The norm in the baseline is to use `!` instead of `not`.  Here you can also say 

    unless feedback_condition.is_feedback_required_for_credit