Try to control growth of duplicate grader sets

Whenever an essay is a duplicate of an essay already in the system, it is held until the original is graded.  Once the original is graded, the graders are appended to the duplicate and the duplicate is returned to the user.

In the demo course, lots of students were pasting in a common essay.  Due to the way gradings work, each iteration of this copying was increasing the number of graders.  So, some submissions were ending up with 100+ graders after several iterations.

This caused mysql issues with large transactions timing out (trying to grab all the graders to post back to LMS).

This PR adds in some measures to avoid the issue, and also adds in a management command to delete excess duplicates.