don't unset front page settings on course copy/import

test plan:
* create a course with wiki pages
* set one of them as the front page

* create another course and copy it into the first course,
* the front page in the original course should stay the same
 as before

* unset the front page
* now copy or import a course package that has a front page
* the new page should become a front page

fixes #CNVS-12137

Change-Id: I91b70ce32cb3bacd5e42d161019b304eca3586bd
Reviewed-on: https://gerrit.instructure.com/34328
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>