fix course copy wiki front page assignment

test plan:
* create a course with a front page
* create a new empty course
* copy from the first course to the second
* should set the front page

* also should not override an already existing
 front page

fixes #CNVS-13398

Change-Id: Iaeb73804d99f362129e3418106d3d9afd5cb4922
Reviewed-on: https://gerrit.instructure.com/35817
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>