Use explicit jQuery import, remove unused eslint globals (#18435)

- Don't rely on globals (window.$) for jQuery import
- Remove eslint globals no longer in use

web_src/js/features/repo-migration.js

@@ -1,3 +1,5 @@
+import $ from 'jquery';
+
 const $service = $('#service_type');
 const $user = $('#auth_username');
 const $pass = $('#auth_password');