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/issue/labels/label.tmpl

@@ -1,5 +1,5 @@
 <a
-	class="item {{if not .label.IsChecked}}hide{{end}}"
+	class="item {{if not .label.IsChecked}}gt-hidden{{end}}"
 	id="label_{{.label.ID}}"
 	href="{{.root.RepoLink}}/{{if or .root.IsPull .root.Issue.IsPull}}pulls{{else}}issues{{end}}?labels={{.label.ID}}"{{/* FIXME: use .root.Issue.Link or create .root.Link */}}
 >

templates/repo/issue/labels/labels_sidebar.tmpl

@@ -1,5 +1,5 @@
 <div class="ui labels list">
-	<span class="no-select item {{if .ctx.HasSelectedLabel}}hide{{end}}">{{.ctx.locale.Tr "repo.issues.new.no_label"}}</span>
+	<span class="no-select item {{if .ctx.HasSelectedLabel}}gt-hidden{{end}}">{{.ctx.locale.Tr "repo.issues.new.no_label"}}</span>
 	<span class="labels-list">
 		{{range .ctx.Labels}}
 			{{template "repo/issue/labels/label" dict "root" $.root "label" .}}