refactor VersionCheckImage class to ES module syntax

app/assets/javascripts/dispatcher.js

@@ -53,6 +53,7 @@ import UsersSelect from './users_select';
 import RefSelectDropdown from './ref_select_dropdown';
 import GfmAutoComplete from './gfm_auto_complete';
 import ShortcutsBlob from './shortcuts_blob';
+import VersionCheckImage from './version_check_image';
 import Wikis from './wikis';
 import ZenMode from './zen_mode';
 import initSettingsPanels from './settings_panels';
@@ -380,7 +381,7 @@ import OAuthRememberMe from './oauth_remember_me';
           new BlobViewer();
           break;
         case 'help:index':
-          gl.VersionCheckImage.bindErrorEvent($('img.js-version-status-badge'));
+          VersionCheckImage.bindErrorEvent($('img.js-version-status-badge'));
           break;
         case 'search:show':
           new Search();

app/assets/javascripts/main.js

@@ -159,7 +159,6 @@ import './user';
 import './user_tabs';
 import './username_validator';
 import './users_select';
-import './version_check_image';
 
 // eslint-disable-next-line global-require, import/no-commonjs
 if (process.env.NODE_ENV !== 'production') require('./test_utils/');

app/assets/javascripts/version_check_image.js

@@ -3,6 +3,3 @@ export default class VersionCheckImage {
     imageElement.off('error').on('error', () => imageElement.hide());
   }
 }
-
-window.gl = window.gl || {};
-gl.VersionCheckImage = VersionCheckImage;