set aria-hidden=true when closing modal dialog

because aria-hidden=false set by g/24502 overrides display: none
as far as JAWS is concerned

test plan:
 - go to the modules page
 - click the "+ Module" button, then cancel out
 - ensure JAWS doesn't still show the "Add Module" and "Cancel"
   buttons in the now-closed modal

fixes CNVS-26508

Change-Id: I6c103ee242922737d676b5e570dce1f222e58fec
Reviewed-on: https://gerrit.instructure.com/74896
Tested-by: Jenkins
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

public/javascripts/vendor/jqueryui/dialog-unpatched.js

@@ -230,6 +230,7 @@ $.widget("ui.dialog", {
 		}
 
 		this._isOpen = false;
+		this.uiDialog.attr('aria-hidden', true);
 
 		if ( this.overlay ) {
 			this.overlay.destroy();