Fix up bug with the way we hide list items

If we use an #unless condition, then when the view is removed from the
template, it is destroyed, and cannot be inserted again. So we’ll just
keep the item there the whole time, and toggle its visibility with CSS.

ember/app/components/discussion/post-header/toggle.js

@@ -9,9 +9,10 @@ var precompileTemplate = Ember.Handlebars.compile;
  */
 export default Ember.Component.extend({
   tagName: 'li',
+  classNameBindings: ['hidden'],
   layout: precompileTemplate('<a href="#" class="btn btn-default btn-more" {{action "toggle"}}>{{fa-icon "ellipsis-h"}}</a>'),
 
-  hideItem: Ember.computed.not('post.isHidden'),
+  hidden: Ember.computed.not('post.isHidden'),
 
   actions: {
     toggle: function() {