show 'everyone else' option when all sections are covered by overrides

even if all the sections are covered by overrides, 'everyone' (or 'everyone
else') should still be an option in the dropdowns, allowing you to move from
full override coverate to just partial override coverage.

fixes CNVS-13054

test plan:
- setup: course with multiple sections
- create an assignment with overrides for all sections
- save, and then return to the edit page
- 'everyone else' should still an option in the dropdowns
- also do a basic regression test of creating/editing assignments with
  different combinations of overrides
- make sure to double check an assignment in a course with no sections

Change-Id: Ifd057ba68049515d7d6414706ec29996b3e0bd6a
Reviewed-on: https://gerrit.instructure.com/36055
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>