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/repo/settings/deploy_keys.tmpl

@@ -15,7 +15,7 @@
 			</div>
 		</h4>
 		<div class="ui attached segment">
-			<div class="{{if not .HasError}}hide{{end}} mb-4" id="add-deploy-key-panel">
+			<div class="{{if not .HasError}}hide{{end}} gt-mb-4" id="add-deploy-key-panel">
 				<form class="ui form" action="{{.Link}}" method="post">
 					{{.CsrfTokenHtml}}
 					<div class="field">