disable duplicate submission comments

fixes CNVS-11958

test plan:

- as an instructor
- go to a submission
- enter a comment
- enter a grade
- click save
- only one instance of the comment should appear

Change-Id: I11f5bdf56bafdbdb40b5cc4ea8bb1e6fc2aca001
Reviewed-on: https://gerrit.instructure.com/36466
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Drew Bowman <dbowman@instructure.com>