Show the mismatched ROOT_URL warning on the sign-in page if OAuth2 is enabled (#25947)

Since OAuth2 will callback the root URL, if the user starts signing in
from a wrong host, Gitea will return 500 because it cannot find the
session.

<details>
<summary>How to reproduce</summary>

<img width="901" alt="image"
src="2c2e255c-e13e-4a11-9be7-b226bee54920">

<img width="1014" alt="image"
src="b31cfcf6-a320-483d-9ce5-ba8562f065e1">

</details>


So show the mismatched ROOT_URL warning on the sign-in page if OAuth2 is
enabled.

<img width="1015" alt="image"
src="99e80b17-c790-49a3-bbf2-2bd9396a7daa">

web_src/js/features/admin/common.js

@@ -10,7 +10,6 @@ export function initAdminCommon() {
   }
 
   // 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