Odd sorting order when some assignments have reduced scoring enabled

[dlglin]

Reduced scoring is not taken into account when sorting sets on the Problem Sets page, which leads to some counterintuitive sorting of open sets.
Consider the following scenario:

• Set A has a reduced scoring date of Wednesday and a due date of Friday.
• Set B has a due date of Thursday, and no reduced scoring period.

In this case Set B will be shown ahead of Set A, though I would argue that Set A has higher urgency.