Add `grading_notes` to problem YAML

[inducer]

Now that rubric has been misappropriated as the template for the grading box, we need someplace else for purely internal notes on grading.

cc @yslan

Related but distinct: #1081

[inducer]

An alternative to this might be to delimit internal notes via some markers, and avoid pasting their content into the grading box. A new top-level item is probably easier/more discoverable.