DEV: Add the @action decorator (#8836)

This also enables`@action` use in plugin connectors.

Setting `actions` earlier allows `setupComponents` to use them, for example, when setting up event listeners.

app/assets/javascripts/discourse-loader.js

@@ -16,6 +16,7 @@ var define, requirejs;
         inject: Ember.inject.controller
       },
       "@ember/object": {
+        action: Ember._action,
         default: Ember.Object,
         get: Ember.get,
         getProperties: Ember.getProperties,