REFACTOR: Replace Ember.controller.inject with import

Puts it more in line with updated ember code

app/assets/javascripts/admin/controllers/modals/admin-add-upload.js.es6

@@ -1,3 +1,4 @@
+import { inject } from '@ember/controller';
 import Controller from "@ember/controller";
 import ModalFunctionality from "discourse/mixins/modal-functionality";
 import { ajax } from "discourse/lib/ajax";
@@ -54,7 +55,7 @@ const SCSS_VARIABLE_NAMES = [
 ];
 
 export default Controller.extend(ModalFunctionality, {
-  adminCustomizeThemesShow: Ember.inject.controller(),
+  adminCustomizeThemesShow: inject(),
 
   uploadUrl: "/admin/themes/upload_asset",