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/user/notification/notification_div.tmpl

@@ -13,7 +13,7 @@
 			{{if and (eq .Status 1)}}
 				<form action="{{AppSubUrl}}/notifications/purge" method="POST" style="margin-left: auto;">
 					{{$.CsrfTokenHtml}}
-					<div class="{{if not $notificationUnreadCount}}hide{{end}}">
+					<div class="{{if not $notificationUnreadCount}}gt-hidden{{end}}">
 						<button class="ui mini button primary" title='{{$.locale.Tr "notification.mark_all_as_read"}}'>
 							{{svg "octicon-checklist"}}
 						</button>