refactor ExportCSVModal into a function and an es module

ee/app/assets/javascripts/export_csv_modal.js

+export default function initExportCSVModal() {
+  const $modal = $('.issues-export-modal');
+  const $downloadBtn = $('.csv_download_link');
+  const $closeBtn = $('.modal-header .close');
+
+  $modal.modal({ show: false });
+  $downloadBtn.on('click', () => $modal.modal('show'));
+  $closeBtn.on('click', () => $modal.modal('hide'));
+}

ee/app/assets/javascripts/issues/export_csv_modal.js

-/* eslint-disable no-new */
-
-$(() => {
-  class ExportCSVModal {
-    constructor() {
-      this.$modal = $('.issues-export-modal');
-      this.$downloadBtn = $('.csv_download_link');
-      this.$closeBtn = $('.modal-header .close');
-      this.init();
-    }
-
-    init() {
-      this.$modal.modal({ show: false });
-      this.$downloadBtn.on('click', () => this.$modal.modal('show'));
-      this.$closeBtn.on('click', () => this.$modal.modal('hide'));
-    }
-  }
-
-  new ExportCSVModal();
-});

ee/app/assets/javascripts/issues/issues_bundle.js

-import './export_csv_modal';
+import initExportCSVModal from 'ee/export_csv_modal';
+
+initExportCSVModal();