Add copy button to markdown code blocks (#17638)

* Add copy button to markdown code blocks

Done mostly in JS because I think it's better not to try getting buttons
past the markup sanitizer.

* add svg module tests

* fix sanitizer regexp

* remove outdated comment

* vertically center button in issue comments as well

* add comment to css

* fix undefined on view file line copy

* combine animation less files

* Update modules/markup/markdown/markdown.go

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

* add test for different sizes

* add cloneNode and add tests for it

* use deep clone

* remove useless optional chaining

* remove the svg node cache

* unify clipboard copy string and i18n

* remove unused var

* remove unused localization

* minor css tweaks to the button

* comment tweak

* remove useless attribute

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/markup/content.js

@@ -1,9 +1,11 @@
 import {renderMermaid} from './mermaid.js';
+import {renderCodeCopy} from './codecopy.js';
 import {initMarkupTasklist} from './tasklist.js';
 
 // code that runs for all markup content
 export function initMarkupContent() {
-  const _promise = renderMermaid(document.querySelectorAll('code.language-mermaid'));
+  renderMermaid();
+  renderCodeCopy();
 }
 
 // code that only runs for comments