Let Plugin-Outlet Render inner template data

- allows passing of template data to the plugin-outlet to be rendered if no plugin is using that slot
 - acts as a fallback when no plugin is found
 - allows wrapping of existing features inside a plugin-outlet that then plugins can overwrite with their own behaviour
 - backwards compatible
 - uses invisible emberviews (as Ember.Component does)
 - the template has full-access to the controller and view, should be totally transparent

app/assets/javascripts/discourse/helpers/plugin-outlet.js.es6

@@ -83,5 +83,15 @@ export default function(connectionName, options) {
       })
     });
     return Ember.Handlebars.helpers.view.call(this, CustomContainerView, options);
+  } else {
+    return Ember.Handlebars.helpers.view.call(this,
+              Ember.View.extend({
+                isVirtual: true,
+                tagName: '',
+                template: function() {
+                  return options.hash.template;
+                }.property()
+              }),
+            options);
   }
 }