add accessiblity to sanitization

fixes CNVS-13697

When you create html markup in canvas, you should be able to add
accessible attributes and roles to elements. This commit allows you to
do that.

Test Plan

As a user that can edit a wiki page
When you edit a wiki page
And you toggle to edit the html
Then you add an aria attribute like aria-label
And you toggle back to the yswyg editor
And you toggle back into the html view
Then aria elements should still be on the html elements
And they should still be there when you save the page and inspect the
elements via developer tools.

Change-Id: Id443170fb3eabcbfcec58fd29b6fcfe1ea71279a
Reviewed-on: https://gerrit.instructure.com/36627
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>