Change method of hiding extra table layer

This module creates a clone of the existing table
as an extra layer which sits on top of it.

This allows the row headers to sit above the table
content when it scrolls.

We were setting `role=presentation` on the extra
table to hide it from assistive technologies.

It seems that this wasn't working. From the spec'
`role-presentation` should remove the semantics of
the table but leave the content.

Testing in Safari, with the OSX Accessibility
Inspector, this isn't happening and the table is
still being reported as a table by the
accessibility API.

This changes to the `aria-hidden` attribute, to
make sure the extra table is ignored.

app/assets/javascripts/fullscreenTable.js

@@ -44,7 +44,7 @@
             .clone()
             .addClass('fullscreen-fixed-table')
             .removeClass('fullscreen-scrollable-table')
-            .attr('role', 'presentation')
+            .attr('aria-hidden', true)
         )
         .append(
           '<div class="fullscreen-right-shadow" />'

tests/javascripts/fullscreenTable.test.js

@@ -123,7 +123,7 @@ describe('FullscreenTable', () => {
 
       expect(tableFrame).not.toBeNull();
       expect(numberColumnFrame).not.toBeNull();
-      expect(numberColumnFrame.getAttribute('role')).toEqual('presentation');
+      expect(numberColumnFrame.getAttribute('aria-hidden')).toEqual('true');
 
     });