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-issue-content.js

@@ -16,7 +16,7 @@ function showContentHistoryDetail(issueBaseUrl, commentId, historyId, itemTitleH
   ${svg('octicon-x', 16, 'close icon inside')}
   <div class="header gt-df gt-ac gt-sb">
     <div>${itemTitleHtml}</div>
-    <div class="ui dropdown dialog-header-options gt-df gt-ac gt-mr-5 hide">
+    <div class="ui dropdown dialog-header-options gt-df gt-ac gt-mr-5 gt-hidden">
       ${i18nTextOptions}${svg('octicon-triangle-down', 14, 'dropdown icon')}
       <div class="menu">
         <div class="item red text" data-option-item="delete">${i18nTextDeleteFromHistory}</div>
@@ -62,7 +62,7 @@ function showContentHistoryDetail(issueBaseUrl, commentId, historyId, itemTitleH
         $dialog.find('.comment-diff-data').removeClass('is-loading').html(resp.diffHtml);
         // there is only one option "item[data-option-item=delete]", so the dropdown can be entirely shown/hidden.
         if (resp.canSoftDelete) {
-          $dialog.find('.dialog-header-options').removeClass('hide');
+          $dialog.find('.dialog-header-options').removeClass('gt-hidden');
         }
       });
     },