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/lib/modal.js

@@ -21,9 +21,9 @@ window.Modal = new class {
     }
   }
 
-  open(modalName, options) {
+  open(modalName, { onCloseGoTo = ''}) {
     this._currentModal.set(modalName);
-    this._onCloseGoTo = options && options.onCloseGoTo || '';
+    this._onCloseGoTo = onCloseGoTo;
   }
 };