allow h1 tags in html sanitation

test plan:
* add html content including one or more h1 tags
 (e.g. "<h1>Header</h1>")
* confirm that it retains the tags rather than
 stripping them out

fixes #CNVS-12321

Change-Id: I74dd56aced493255b2f8a5834d2a5a841de6d910
Reviewed-on: https://gerrit.instructure.com/33139
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Florence <ryanf@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Ryan Florence <ryanf@instructure.com>