remove sideeffects, init per page

app/assets/javascripts/milestone.js

@@ -5,7 +5,6 @@ import { mouseenter, debouncedMouseleave, togglePopover } from './shared/popover
 export default class Milestone {
   constructor() {
-    Milestone.initDeprecationMessage();
     this.bindTabsSwitching();
     // Load merge request tab if it is active

app/assets/javascripts/pages/dashboard/milestones/show/index.js

@@ -6,4 +6,6 @@ document.addEventListener('DOMContentLoaded', () => {
   new Milestone(); // eslint-disable-line no-new
   new Sidebar(); // eslint-disable-line no-new
   new MountMilestoneSidebar(); // eslint-disable-line no-new
+
+  Milestone.initDeprecationMessage();
 });

app/assets/javascripts/pages/groups/milestones/show/index.js

 import initMilestonesShow from '~/pages/milestones/shared/init_milestones_show';
-document.addEventListener('DOMContentLoaded', initMilestonesShow);
+import Milestone from '~/milestone';
+document.addEventListener('DOMContentLoaded', () => {
+  initMilestonesShow();
+
+  Milestone.initDeprecationMessage();
+});