fix ally issue with icon on moderated grade page

fixes CNVS-25201

After an assignments grades have been posted via moderated grades,
there is an attention icon that shows up. The screenreader picks this
icon up in screenreaders. We want to hide this.

Test Plan
Set up an assignment for moderated grading
Post the grades with reviewers
With a screenreader in safari (VO) navigate to the attention alert box
The alert text should be read, however the 'i' icon should be invisible
to screenreaders

Change-Id: If8f3e48ef9565eba6e2f4cab142f783492765c94
Reviewed-on: https://gerrit.instructure.com/67739
Tested-by: Jenkins
Reviewed-by: Clay Diffrient <[email protected]>
QA-Review: Jahnavi Yetukuri <[email protected]>
Product-Review: Sterling Cobb <[email protected]>

app/jsx/assignments/ModerationHeader.jsx

@@ -24,7 +24,7 @@ define([
         return (
 
           <div className="ic-notification">
-            <div className="ic-notification__icon" role="presentation">
+            <div className="ic-notification__icon" aria-hidden='true' role="presentation">
               <i className="icon-info"></i>
             </div>
             <div className="ic-notification__content">