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>

web_src/js/features/repo-findfile.js

@@ -88,7 +88,7 @@ function filterRepoFiles(filter) {
     const $row = $(tmplRow);
     const $a = $row.find('a');
     $a.attr('href', `${treeLink}/${escapePath(r.matchResult.join(''))}`);
-    const $octiconFile = $(svg('octicon-file')).addClass('mr-3');
+    const $octiconFile = $(svg('octicon-file')).addClass('gt-mr-3');
     $a.append($octiconFile);
     // if the target file path is "abc/xyz", to search "bx", then the matchResult is ['a', 'b', 'c/', 'x', 'yz']
     // the matchResult[odd] is matched and highlighted to red.