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* remove outdated comment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>

templates/base/head.tmpl

@@ -46,6 +46,10 @@
 			]).values()),
 			{{end}}
 			mermaidMaxSourceCharacters: {{MermaidMaxSourceCharacters}},
+			i18n: {
+				copy_success: '{{.i18n.Tr "copy_success"}}',
+				copy_error: '{{.i18n.Tr "copy_error"}}',
+			}
 		};
 	</script>
 	<link rel="icon" href="{{AssetUrlPrefix}}/img/logo.svg" type="image/svg+xml">