Only use course dates to determine soft-concluded state if set to override term dates

Fixes CNVS-6541

Test plan:
- Create a course with a course end date in the past, but belonging to a current term
- Verify course dates do not override term dates
- Course should not be shown as soft-concluded
-- (can be seen in /courses/:course_id/users, on the 'add people' button)

Change-Id: I3c22b57192341977a3b9694af9b4c2ecda2be413
Reviewed-on: https://gerrit.instructure.com/39438
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>