Specify Completion criteria on per-step basis

[hirokiterashima]

If the author of a step can specify what it means for a step to be "complete", we can provide a better idea of progress for both the student and the teacher.

For example, a C-Rater item that requires student to submit to C-Rater twice: once for the initial scoring, and the second time after they've revised. Currently, we mark the step as completed immediately after the first submission. This is a bit misleading because really the student has not fully completed what they are required to do.