De-dupe assignments

[hcientist]

It should not be possible (as is currently the case in prod 😅) for a teacher to assign the same activity to the same student for the same piece.

Proposed resolution:

1. Update the assignment model to have this constraint (unique across 3 assignment fields: activity, enrollment, piece)
2. makemigrations
   1. update the autogenerated migration to have a cleanup process that runs first (before the operation that adds the unique constraint) which will:
      1. for each enrollment
         1. for each piece
            1. for each activity
               1. determine if there are "duplicate" assignments (aggregate a "dupe_set")
                  1. if there are, try to see whether more than 1 has a submission:
                     1. if there is exactly 1 assignment in the "dupe_set" with submissions, delete all other assignments in the "dupe_set"
                     2. elif there are 0 with submissions, delete all but the most recent assignments in the "dupe_set"
                     3. elif there are > 1 with submissions, find the one with the most recent submission or submissionattachment and keep this and delete the others.

see the de-dupe-assns branch