add red dots to duplicate assignment names

fixes SIS-382

Test plan:
- Create a few assignments
- Make two or more assignments the exact same name
- Make one or more of the assignments not have dates
- Go to the gradebook
- Click on 'Publish Grades'
- Confirm the duplicate assignments have the red dot next to them
- Confirm the duplicate assingments that have no dates have the
  red dots for both the name and the date
- Change one name and confirm the dot/dots disappear
- Confirm the date dots remain there
- Add dates and confirm that the assignments are accepted and
  disappear
- Confirm normal functionality

Change-Id: I355dfac5348e89ed08bd9030ae8b694f276cc209
Reviewed-on: https://gerrit.instructure.com/43879
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jayce Higgins <jhiggins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Nick Houle <nhoule@instructure.com>