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

templates/repo/settings/deploy_keys.tmpl

@@ -15,7 +15,7 @@
 			</div>
 		</h4>
 		<div class="ui attached segment">
-			<div class="{{if not .HasError}}hide{{end}} gt-mb-4" id="add-deploy-key-panel">
+			<div class="{{if not .HasError}}gt-hidden{{end}} gt-mb-4" id="add-deploy-key-panel">
 				<form class="ui form" action="{{.Link}}" method="post">
 					{{.CsrfTokenHtml}}
 					<div class="field">