Prefer ES5 methods over underscore utilities

Since 07cc454 (ie the switch to Meteor 1.2) we includes the `es5-shim`
polyfill to support methods like `Array.prototype.forEach` in a
consistent way across all supported browsers (IE8+).

MDG recently released a blog post recommending the use of these native
methods instead of underscore [0]. We know follow this recommendation.

This commit also favor some ES6 features (argument defaults,
destructing assignment) in places where we didn’t use them.

[0]: http://info.meteor.com/blog/es2015-get-started

client/components/sidebar/sidebar.js

@@ -95,10 +95,10 @@ BlazeComponent.extendComponent({
   events() {
     // XXX Hacky, we need some kind of `super`
     const mixinEvents = this.getMixin(Mixins.InfiniteScrolling).events();
-    return mixinEvents.concat([{
+    return [...mixinEvents, {
       'click .js-toggle-sidebar': this.toggle,
       'click .js-back-home': this.setView,
-    }]);
+    }];
   },
 }).register('sidebar');