Only get milestone when EE

Create boards_util to conditionally `getMilestone` when in EE

app/assets/javascripts/boards/boards_util.js

+export function getMilestone() {
+  return null;
+}
+
+export default {
+  getMilestone,
+};

app/assets/javascripts/boards/components/board_new_issue.vue

 <script>
 import $ from 'jquery';
 import { GlButton } from '@gitlab/ui';
+import { getMilestone } from 'ee_else_ce/boards/boards_util';
 import eventHub from '../eventhub';
 import ProjectSelect from './project_select.vue';
 import ListIssue from '../models/issue';
@@ -51,11 +52,14 @@ export default {
 
       const labels = this.list.label ? [this.list.label] : [];
       const assignees = this.list.assignee ? [this.list.assignee] : [];
+      const milestone = getMilestone(this.list);
+
       const issue = new ListIssue({
         title: this.title,
         labels,
         subscribed: true,
         assignees,
+        milestone,
         project_id: this.selectedProject.id,
       });