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* remove outdated comment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>

package.json

@@ -51,6 +51,7 @@
     "eslint-plugin-vue": "8.0.3",
     "jest": "27.3.1",
     "jest-extended": "1.1.0",
+    "jest-raw-loader": "1.0.1",
     "postcss-less": "5.0.0",
     "stylelint": "14.0.1",
     "stylelint-config-standard": "23.0.0",