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">
Close#22847
This PR:
* introduce Gitea's own `showElem` and related functions
* remove jQuery show/hide
* remove .hide class
* remove inline style=display:none
From now on:
do not use:
* "[hidden]" attribute: it's too weak, can not be applied to an element
with "display: flex"
* ".hidden" class: it has been polluted by Fomantic UI in many cases
* inline style="display: none": it's difficult to tweak
* jQuery's show/hide/toggle: it can not show/hide elements with
"display: xxx !important"
only use:
* this ".gt-hidden" class
* showElem/hideElem/toggleElem functions in "utils/dom.js"
cc: @silverwind , this is the all-in-one PR
