Improve AppUrl/ROOT_URL checking (#22836)

After some PRs:
* #21986
* #22795
* #22808
* #22831
* #22839

Users won't be affected by the ROOT_URL problem in most cases. Close
#19345

This PR improves AppUrl/ROOT_URL checking, only check it on the admin
page, and the message is also updated.

Feel free to suggest about more English-native messages.



![image](https://user-images.githubusercontent.com/2114189/217811809-7d44ddb7-2c4a-46d0-a5db-8ae6ee65f8c3.png)

web_src/js/features/admin/common.js

@@ -1,12 +1,17 @@
 import $ from 'jquery';
+import {checkAppUrl} from '../common-global.js';
 
 const {csrfToken} = window.config;
 
 export function initAdminCommon() {
-  if ($('.admin').length === 0) {
+  if ($('.page-content.admin').length === 0) {
     return;
   }
 
+  // check whether appUrl(ROOT_URL) is correct, if not, show an error message
+  // only admin pages need this check because most templates are using relative URLs now
+  checkAppUrl();
+
   // New user
   if ($('.admin.new.user').length > 0 || $('.admin.edit.user').length > 0) {
     $('#login_type').on('change', function () {