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.
Since `feedback_viewing_penalty` is an integer, dividing by another integer will do integer division, e.g. `12/100 == 0`.  To force the division we expect, divide the attribute by a float: `feedback_viewing_penalty/100.0`.