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/issue/view_content/update_branch_by_merge.tmpl

@@ -8,7 +8,7 @@
 		</div>
 		<div class="item-section-right">
 			{{if and .UpdateAllowed .UpdateByRebaseAllowed}}
-				<div class="dib">
+				<div class="gt-dib">
 					<div class="ui buttons update-button">
 						<button class="ui button" data-do="{{.Link}}/update" data-redirect="{{.Link}}">
 							<span class="button-text">