allow video and audio html tags in html content

test plan:
* try to add audio and video tags in html content
* should not remove the tags when saving

closes #CNVS-9660

Change-Id: I46e363d2150f16a07ec8cf096be8b7652c437254
Reviewed-on: https://gerrit.instructure.com/36594
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>