accessibility: Add aria-hidden attribute to the help modal (LMS-584)

Added a 'aria-hidden="true"' attribute to the modal initially, then
toggle the value of that attribute to "false" when the modal is shown.
Also remove the attribute when the modal is closed, whether it's through
a click on the close button or on the overlay.

lms/templates/help_modal.html

@@ -12,7 +12,7 @@
   <a href="#help-modal" rel="leanModal">${_("Help")}</a>
 </div>
 
-<section id="help-modal" class="modal">
+<section id="help-modal" class="modal" aria-hidden="true">
   <div class="inner-wrapper" id="help_wrapper">
     <header>
       <h2>${_('{span_start}{platform_name}{span_end} Help').format(span_start='<span class="edx">', span_end='</span>', platform_name=settings.PLATFORM_NAME)}</h2>
@@ -127,6 +127,11 @@ discussion_link = get_discussion_link(course) if course else None
 
 <script type="text/javascript">
 (function() {
+    var onModalClose = function() {
+        $(".help-modal .close-modal").off("click");
+        $("#lean_overlay").off("click");
+        $("#help-modal").attr("aria-hidden", "true");
+    };
     $(".help-tab").click(function() {
         $(".field-error").removeClass("field-error");
         $("#feedback_form")[0].reset();
@@ -135,6 +140,9 @@ discussion_link = get_discussion_link(course) if course else None
         $("#feedback_error").css("display", "none");
         $("#feedback_success_wrapper").css("display", "none");
         $("#help_wrapper").css("display", "block");
+        $("#help-modal").attr("aria-hidden", "false");
+        $("#help-modal .close-modal").click(onModalClose);
+        $("#lean_overlay").click(onModalClose);
     });
     showFeedback = function(event, issue_type, title, subject_label, details_label) {
         $("#help_wrapper").css("display", "none");