refactor UsernameValidator class to ES module syntax

app/assets/javascripts/dispatcher.js

 /* eslint-disable func-names, space-before-function-paren, no-var, prefer-arrow-callback, wrap-iife, no-shadow, consistent-return, one-var, one-var-declaration-per-line, camelcase, default-case, no-new, quotes, no-duplicate-case, no-case-declarations, no-fallthrough, max-len */
-/* global UsernameValidator */
 /* global ActiveTabMemoizer */
 /* global ShortcutsNavigation */
 /* global IssuableIndex */
@@ -53,6 +52,7 @@ import UsersSelect from './users_select';
 import RefSelectDropdown from './ref_select_dropdown';
 import GfmAutoComplete from './gfm_auto_complete';
 import ShortcutsBlob from './shortcuts_blob';
+import UsernameValidator from './username_validator';
 import VersionCheckImage from './version_check_image';
 import Wikis from './wikis';
 import ZenMode from './zen_mode';

app/assets/javascripts/main.js

@@ -157,7 +157,6 @@ import './tree';
 import './usage_ping';
 import './user';
 import './user_tabs';
-import './username_validator';
 import './users_select';
 
 // eslint-disable-next-line global-require, import/no-commonjs

app/assets/javascripts/username_validator.js

@@ -8,7 +8,7 @@ const successMessageSelector = '.username .validation-success';
 const pendingMessageSelector = '.username .validation-pending';
 const invalidMessageSelector = '.username .gl-field-error';
 
-class UsernameValidator {
+export default class UsernameValidator {
   constructor() {
     this.inputElement = $('#new_user_username');
     this.inputDomElement = this.inputElement.get(0);
@@ -129,5 +129,3 @@ class UsernameValidator {
     $inputErrorMessage.show();
   }
 }
-
-window.UsernameValidator = UsernameValidator;