Default course: Add _canShowCorrectness properties

### Subject of the issue
The `_canShowCorrectness` [property](https://github.com/adaptlearning/adapt-contrib-mcq/blob/4fffe4a050dbeb2ef85cc0e7ef0e6700249088c6/example.json#L20) should be added to question components in the [default course](https://github.com/adaptlearning/adapt_framework/tree/master/src/course). The default should be `false`.