fix wiki links in course copy/migrations

test plan:
 - with draft state enabled
 * create a wiki page
   - use the sidebar to add a link to another wiki page
 * save the page
 * edit the page
   - html view should show the href as ../pages/.. not ../wiki/..
   - copy the link, changing the href to be ../wiki/..
 * save the page
 * copy the course
 * navigate to the copied wiki page
   - both links should point to the page in the new course

fixes CNVS-11443

Change-Id: Ib7f8b175ac6d894a9bfa651f8536b7cd48b60aac
Reviewed-on: https://gerrit.instructure.com/30780
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>