Extract mounting of multiple boards switcher

app/assets/javascripts/boards/index.js

 import $ from 'jquery';
 import Vue from 'vue';
 
+import mountMultipleBoardsSwitcher from 'ee_else_ce/boards/mount_multiple_boards_switcher';
 import Flash from '~/flash';
 import { __ } from '~/locale';
 import './models/label';
@@ -278,4 +279,6 @@ export default () => {
       `,
     });
   }
+
+  mountMultipleBoardsSwitcher();
 };

app/assets/javascripts/boards/mount_multiple_boards_switcher.js

+// this will be moved from EE to CE as part of https://gitlab.com/gitlab-org/gitlab-ce/issues/53811
+export default () => {};