[Speedgrader] Reduce friction for complete/incomplete assignments

Problem statement:

Grading a complete/incomplete assignment currently requires two clicks (once on the combo box, once on "complete", "incomplete", or "excused"). This unnecessarily slows down grading and increases potential misclicks.

Proposed solution:

Displaying three buttons ("complete", "incomplete", and "excused") instead of the combo box reduces the required clicks in half, increasing grading speed and reducing one option for mis-clicks. The currently selected grade would need to be indicated (such as by bolding the text) when revisiting the assignment.

User role(s):

instructor