don't sanitize title attributes in html content

test plan:
* try to save html content with a title attibute on an
 html element other than a or img:
 e.g. "<p title='a title'>text</p>"
* the attribute should not be removed

closes #CNVS-8357

Change-Id: Ib6bf534a75aa30fff8efc6345189114ff3718e1c
Reviewed-on: https://gerrit.instructure.com/36590
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>