fix processing of user content through pages api

test plan:
 * create a wiki page
   - include a media comment
 * query the page body, using the api
   - GET ../pages/page-url
   - body should include audio/video tag (depending on comment type)
 * update the page body, using the api
   - PUT ../pages/page-url
   - make a minor change to the queried page body (e.g. add a word)
 * navigate to the page in a browser
   - the page should _not_ render the media comment with audio/video tags

fixes CNVS-11427

Change-Id: Icf3f2b48220b92d2ce19729ef7784cb38de5d328
Reviewed-on: https://gerrit.instructure.com/30714
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>