Replace some gt- classes with tw- (#29570)

Replace 18 `gt-` prefixes with `tw-` with perl replacement. I manually
checked them all with `rg` afterwards.

(cherry picked from commit a2e90014ec20a1085449a66061389cfe0d12260f)

Conflicts:
	templates/repo/header.tmpl
	because some of the header moved to header_fork.tmpl

web_src/js/features/common-global.js

@@ -234,7 +234,7 @@ export function initDropzone(el) {
         // 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 = 'gt-text-center';
+        copyLinkElement.className = 'tw-text-center';
         // 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', async (e) => {