Fixed #31493 -- Replaced var with const and let keywords in JavaScript.

The eslint configuration and the admin script compress.py have been
updated for ES6.

The unused fallback of globals.django in jquery.init.js was removed. It
is always included before jsi18n-mocks.test.js and it always sets the
global value.

js_tests/admin/actions.test.js

@@ -9,7 +9,7 @@ QUnit.module('admin.actions', {
         window._actions_icnt = '100';
         /* eslint-enable */
 
-        var $ = django.jQuery;
+        const $ = django.jQuery;
         $('#qunit-fixture').append($('#result-table').text());
 
         $('tr input.action-select').actions();
@@ -17,7 +17,7 @@ QUnit.module('admin.actions', {
 });
 
 QUnit.test('check', function(assert) {
-    var $ = django.jQuery;
+    const $ = django.jQuery;
     assert.notOk($('.action-select').is(':checked'));
     $('#action-toggle').click();
     assert.ok($('.action-select').is(':checked'));