Fix ChannelSettingsMixin

Fixes broken notifications.
Regression introduced in 25751bc: service injections are computed
properties...

src/app/mixins/ChannelSettingsMixin.js

@@ -1,13 +1,15 @@
 define( [ "ember" ], function( Ember ) {
 
+	var get = Ember.get;
+
 	return Ember.Mixin.create({
 		/**
 		 * Load channel specific settings
 		 * @param {number} id
 		 * @returns {Promise}
 		 */
 		loadChannelSettings: function( id ) {
-			var store = this.store;
+			var store = get( this, "store" );
 			return store.find( "channelSettings", id )
 				.then(function( record ) {
 					// get its data and unload it