Refactor hiding-methods, remove jQuery show/hide, remove .hide class, remove inline style=display:none (#22950)

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

web_src/js/features/repo-findfile.js

@@ -1,5 +1,6 @@
 import $ from 'jquery';
 import {svg} from '../svg.js';
+import {toggleElem} from '../utils/dom.js';
 
 const {csrf} = window.config;
 
@@ -83,7 +84,7 @@ function filterRepoFiles(filter) {
   const filterResult = filterRepoFilesWeighted(files, filter);
   const tmplRow = `<tr><td><a></a></td></tr>`;
 
-  $repoFindFileNoResult.toggle(filterResult.length === 0);
+  toggleElem($repoFindFileNoResult, filterResult.length === 0);
   for (const r of filterResult) {
     const $row = $(tmplRow);
     const $a = $row.find('a');