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/common-global.js

@@ -172,7 +172,7 @@ export function initGlobalDropzone() {
           // Create a "Copy Link" element, to conveniently copy the image
           // or file link as Markdown to the clipboard
           const copyLinkElement = document.createElement('div');
-          copyLinkElement.className = 'tc';
+          copyLinkElement.className = 'gt-tc';
           // The a element has a hardcoded cursor: pointer because the default is overridden by .dropzone
           copyLinkElement.innerHTML = `<a href="#" style="cursor: pointer;">${svg('octicon-copy', 14, 'copy link')} Copy link</a>`;
           copyLinkElement.addEventListener('click', (e) => {