Move helpers to be prefixed with gt- (#22879)

As discussed in #22847 the helpers in helpers.less need to have a
separate prefix as they are causing conflicts with fomantic styles

This will allow us to have the `.gt-hidden { display:none !important; }`
style that is needed to for the reverted PR.

Of note in doing this I have noticed that there was already a conflict
with at least one chroma style which this PR now avoids.

I've also added in the `gt-hidden` style that matches the tailwind one
and switched the code that needed it to use that.

Signed-off-by: Andrew Thornton <art27@cantab.net>

---------

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/status/404.tmpl

@@ -1,5 +1,5 @@
 {{template "base/head" .}}
-<div role="main" aria-label="{{.Title}}" class="page-content ui container center full-screen-width {{if .IsRepo}}repository{{end}}">
+<div role="main" aria-label="{{.Title}}" class="page-content ui container center gt-full-screen-width {{if .IsRepo}}repository{{end}}">
 	{{if .IsRepo}}{{template "repo/header" .}}{{end}}
 	<div class="ui container center">
 		<p style="margin-top: 100px"><img src="{{AssetUrlPrefix}}/img/404.png" alt="404"/></p>

templates/status/500.tmpl

@@ -1,5 +1,5 @@
 {{template "base/head" .}}
-<div role="main" aria-label="{{.Title}}" class="page-content ui container full-screen-width center">
+<div role="main" aria-label="{{.Title}}" class="page-content ui container gt-full-screen-width center">
 	<p style="margin-top: 100px"><img src="{{AssetUrlPrefix}}/img/500.png" alt="500"/></p>
 	<div class="ui divider"></div>
 	<br>