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* remove outdated comment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/features/common-global.js

@@ -104,7 +104,7 @@ export function initGlobalCommon() {
   $('.ui.progress').progress({
     showActivity: false
   });
-  $('.poping.up').popup();
+  $('.poping.up').attr('data-variation', 'inverted tiny').popup();
   $('.top.menu .poping.up').popup({
     onShow() {
       if ($('.top.menu .menu.transition').hasClass('visible')) {