fix wiki sidebar file link translation in api content

test plan:
* add an inlineable file to a course (e.g. a '.html' file)
* create a wiki page content and add a link to the file through
 the wiki sidebar
* following the link on the saved page should take you
 to a preview page for the inlined content

fixes #CNVS-14211

Change-Id: I14d2fc6d90f893086eff5498d0c670b9251fdddd
Reviewed-on: https://gerrit.instructure.com/39191
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>