Assign Diff Tags to Individual Assignments

closes EGG-472
flag=assign_to_differentiation_tags

This adds necessary UI changes to allow users
to create overrides involving Differentiation tags
on individual assignments

Test Plan:
- Enable the following feature flags:
 - Account Level -> Assign to Differentiation Tags
 - Course Level -> Differentiation Tags
 - Account Setting -> Allow Assign to Differentiation Tags
- Go to a course and create a non-collaborative group set
 - This can be done via Graphiql using the 'createGroupSet'
   mutation
 - Make sure to add groups to the groupset when creating them
  - createGroupCount: 3
- Create an Assignment
- On that assignment's page
 - Click the "Assign To" button
 - Add an override for a differentiation tag
 - Click save
 - Reopen the "Assign To" tray and verify that the override
   persisted
- Go to the Modules page
 - Add the new assignment to a module
 - Select the kebab menu on the assignment
 - Select "Assign To"
 - Verify that the override for the Differentiation Tag
   is shown in the tray

Change-Id: Id4dfdc108a9018cbe44e81cbaf64f9d18332bc58
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/369478


Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Chrystal Langston <chrystal.langston@instructure.com>
QA-Review: Chrystal Langston <chrystal.langston@instructure.com>
Product-Review: Sam Garza <sam.garza@instructure.com>