Return completed submissions to peer-grader if no pending ones

@VikParuchuri , maybe @brianhw 

This is solves one of our main anticipated blockers for launching fall courses.
For each assignment, we wanted to set the "number of peer grades a student is supposed to give" to be 1 greater than the "number of peer grades a student submission should receive".  This will help with the "not enough peer grades per submission" problem. 

However, this setting creates its own problem, which is that if everyone does their job in giving peer assessments, there won't be any "pending" submissions left for anyone to grade.  So everyone is deadlocked.

This PR is designed to solve this issue by giving peer graders "finished" submissions to grade.  It also resets the `posted_back_to_xqueue` parameter on the submission so that the latest evaluation will get posted by to xqueue and lms.