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/settings/keys_principal.tmpl

@@ -34,7 +34,7 @@
 	</div>
 	<br>
 
-	<div {{if not .HasPrincipalError}}class="hide"{{end}} id="add-ssh-principal-panel">
+	<div {{if not .HasPrincipalError}}class="gt-hidden"{{end}} id="add-ssh-principal-panel">
 		<h4 class="ui top attached header">
 			{{.locale.Tr "settings.add_new_principal"}}
 		</h4>