restrict former self-enrollment enabled courses through account

test plan:
* enable self-enrollment on the root account
* enable self-enrollment on a course
* disable self-enrollment on the root account
* self-enrollment should not work on the course

closes #CNVS-9109

Change-Id: Ic186497ae11376c00872200460d9850e4dfecc38
Reviewed-on: https://gerrit.instructure.com/39684
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>