Hide "check all/none" buttons when JavaScript isn't available

These buttons only work without JavaScript, so we shouldn't show them in
this case.

I was wondering whether we should use the `hidden` HTML attribute so
these buttons don't show up when stylesheets haven't loaded either. Not
doing so because we already have a stylesheet for the <noscript>
scenario. We might change our minds regarding how to handle these styles
in the future.

app/assets/stylesheets/noscript.scss

@@ -3,3 +3,7 @@
     display: block;
   }
 }
+
+.check-all-none {
+  display: none;
+}