set focus for assignment muter dialogs

fixes CNVS-10555

test plan:
 In screenreader gradebook and gradebook2
  - open the assignment muter dialog
  - focus should be on the content of the dialog

Change-Id: Id4972ebef2f9a496b984bd0e799172844ffbb158
Reviewed-on: https://gerrit.instructure.com/29309
Reviewed-by: Matthew Irish <mirish@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>

app/coffeescripts/AssignmentMuter.coffee

@@ -27,6 +27,7 @@ define [
           click: =>
             @$dialog.disableWhileLoading $.ajaxJSON(@url, 'put', { status : true }, @afterUpdate)
         }]
+        open: => setTimeout (=> @$dialog.find('#assignment_muter_content').focus()), 100
         close: => @$dialog.remove()
         resizable: false
         width: 400

app/views/jst/mute_dialog.handlebars

@@ -1,4 +1,5 @@
 <div title="{{#t "mute_assignment"}}Mute Assignment{{/t}}">
+  <div id="assignment_muter_content" tabindex="-1">
     <p>
       {{#t "sure_you_want_to_mute"}}
         Are you sure you want to mute this assignment? While this assignment
@@ -22,4 +23,5 @@
         notifications again by clicking the "Unmute Assignment" link.
       {{/t}}
     </p>
+  </div>
 </div>