Move EE differences for `app/assets/javascripts/boards/models/milestone.js`

The file app/assets/javascripts/boards/models/milestone.js has differences between CE and EE.

Diferences

diff --git a/home/yorickpeterse/Projects/gitlab/gdk-ce/gitlab/app/assets/javascripts/boards/models/milestone.js b/home/yorickpeterse/Projects/gitlab/gdk-ee/gitlab/app/assets/javascripts/boards/models/milestone.js
index 17d15278a74..5fc9f31eb89 100644
--- a/home/yorickpeterse/Projects/gitlab/gdk-ce/gitlab/app/assets/javascripts/boards/models/milestone.js
+++ b/home/yorickpeterse/Projects/gitlab/gdk-ee/gitlab/app/assets/javascripts/boards/models/milestone.js
@@ -1,7 +1,11 @@
-class ListMilestone {
+export default class ListMilestone {
   constructor(obj) {
     this.id = obj.id;
     this.title = obj.title;
+    this.path = obj.path;
+    this.state = obj.state;
+    this.webUrl = obj.web_url || obj.webUrl;
+    this.description = obj.description;
   }
 }

What needs to be done

1. Use ifEE to wrap the differences