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/wiki/view.tmpl

@@ -47,7 +47,7 @@
 				</div>
 				<div class="eight wide right aligned column">
 					{{if .EscapeStatus.Escaped}}
-						<a class="ui small button unescape-button" style="display: none;">{{.locale.Tr "repo.unescape_control_characters"}}</a>
+						<a class="ui small button unescape-button gt-hidden">{{.locale.Tr "repo.unescape_control_characters"}}</a>
 						<a class="ui small button escape-button">{{.locale.Tr "repo.escape_control_characters"}}</a>
 					{{end}}
 					{{if and .CanWriteWiki (not .Repository.IsMirror)}}